After mortal sin there is nothing that we should avoid more carefully than venial sin. Although it is much less serious than mortal sin, it is nevertheless a moral evil, and moral evil is the greatest of all evils. Before this type of evil all others of the physical order fade away as if they were nothing. Neither sickness nor death itself can be compared to the evil of sin. It is necessary therefore to have clear ideas about the nature, classes, malice and consequences of venial sin so that one can cultivate a horror of it and put into practice the means necessary to avoid it.
Nature and Kinds
This is one of the most difficult questions in all theology. For our purpose, however, it is sufficient to say that, as distinct from mortal sin, venial sin consists in a simple deviation and not a total aversion from the ultimate end. It is a sickness and not the death of the soul. The sinner who commits a mortal sin is like the traveller who, intending to reach a certain point, turns his back on it and begins to travel in the opposite direction. But he who commits a venial sin merely departs from the straight path without abandoning his orientation toward the goal to which he is travelling.
It is possible to distinguish three classes of venial sins:
1) Those sins which by their very nature involve a disorder or deviation, although only a slight one, such as a small lie which does no damage to anyone.
2) Those sins which, although of themselves gravely forbidden, because of the smallness of the matter involved, constitute only a light disorder, as to steal a small amount of money.
3) Those sins which lack complete deliberation orfull consent of the will in matters which would otherwise be serious sins, such as inadvertent or semi‑deliberate impure thoughts.
The mere multiplication of venial sins does not of itself change the species of the sin. A thousand venial sins do not equal a single mortal sin. Nevertheless, a venial sin could become a mortal sin for any one of the following reasons:
a) Because of an erroneous conscience or a seriously doubtful conscience concerning the grave malice of a deliberate act. Thus he who erroneously believes that an action which is objectively only venially sinful is a mortal sin would commit a mortal sin if he performed that action. One would also commit a mortal sin in performing an action if he has serious doubts as to whether or not it is a mortal sin or only a venial sin, for one is obliged to solve such a doubt before performing the action.
b) By reason of an end which is gravely evil, as would occur if one performs an act which is a light sin for the purpose of causing another to commit a serious sin.
c) By reason of the proximate danger of falling into mortal sin if one commits a particular venial sin, as would be the case if one were to let himself become angry when he knows that he will very likely end by inflicting grave damage or injury on his neighbour.
d) By reason of the grave scandal which would be occasioned by the commission of a light sin, e.g., if a venial sin committed by a priest were to become the occasion of a serious sin on the part of a layman.
e) By formal contempt of a law which binds under light obligation. Contempt is called formal if it is directed against authority as such; it is called material if is directed to some other element, such as a disdain for the thing forbidden because one thinks it is of little importance.
f) By the accumulation of material which may increase until it is grave matter.
Malice of Venial Sin
It is certain that there is a great difference between the malice of a mortal sin and that of a venial sin. The Church has condemned the following proposition of Baius: "There is no sin which is venial by its very nature, but every sin merits eternal punishment.” Nevertheless, venial sin does constitute a true offence against God, an effective disobedience of His law, and an act of ingratitude for His great benefits. On the one hand there is the will of God and His glory; on the other, our own desires and selfishness. In the case of venial sin, we in effect choose the latter.
It is true that we should not prefer them if we knew that they would separate us radically from God (and in this we have the distinction between venial and mortal sin, because the latter consists in our turning away from God completely), but It is certain that the lack of respect toward God is of itself very great even in the case of venial sin. St. Teresa says in this regard:
From any sin, however small, committed with full knowledge, may God deliver us, especially since we are sinning against so great a Sovereign and realize that He is watching us. That seems to me to be a sin of malice aforethought; it is as though one were to say: "Lord, although this displeases Thee, I shall do it. I know that Thou seest it and I know that Thou wouldst not have me do it; but although I understand this, I would rather follow my own whim and desire than Thy will." If we commit a sin in this way, however slight, it seems to me that our offense is not small but very, very great.
Nevertheless, it is necessary to distinguish between venial sins committed out of weakness, surprise or lack of advertence and deliberation, and those which are committed coldly and with the complete awareness that one thereby displeases God. We can never completely avoid the former, and God, who knows very well the clay of which we are made, readily forgives us these sins of weakness. The only thing that one can do about these faults is to try to diminish their number as far as possible and to avoid discouragement, which would be fatal for one who is striving for perfection and always presupposes a self‑love which is more or less dissimulated. St. Francis de Sales says in this respect:
Although it is reasonable to feel discouragement and to be sorry for having committed any faults, this discouragement should not be sour, angry, acrimonious or choleric; and this is the great defect of those who, seeing themselves angry, become impatient with their own impatience and become angry at their own anger....
Believe me, Philothea, that just as the sweet and cordial reproaches of a father make more of an impression on a son than his rage and anger, so also, if we reproach our heart when it commits some fault with sweet and peaceful reproaches, using more compassion than anger and arousing the heart to amend, we shall succeed in arousing a repentance which is much more performed and penetrating than that which could be aroused with resentment, anger and anxiety.... Therefore, when your heart falls., raise it sweetly, humbling yourself greatly in the presence of God by the recognition of your misery, without being surprised at your fall; for what is so strange that sickness should be sick, that weakness should be weak, and that misery should be wretched. Nevertheless, detest with all your heart the offense which you have committed against God and, filled with courage and confidence in His mercy, begin again the practice of that virtue which you have abandoned.
If one acts in this way, reacting promptly against those faults of weakness with a profound repentance full of meekness, humility and confidence in the mercy of God, they will leave scarcely any trace in the soul, and they will not constitute a serious obstacle in the path of our sanctification.
But when venial sins are committed coldly, with perfect deliberation and advertence, they constitute an insuperable obstacle to perfection. They make it impossible to proceed along the road to sanctity. Those sins sadden the Holy Ghost, as St. Paul says, and they completely paralyse His sanctifying work in the soul. Father Lallemant says in this regard:
One is astonished to see so many religious who, after having lived forty or fifty years in the state of grace, saying Mass every day and practicing all the holy exercises of the religious life, and, consequently, possessing all the gifts of the Holy Ghost in a very high degree‑one is astonished, I say, to see that these religious give no recognition to the gifts of the Holy Ghost in their acts and in their conduct; to see that their life is completely natural; that, when they are corrected or when they are discouraged, they show their resentment; that they show so much concern for the praise, the esteem and the applause of the world; that they delight in it, and they love and seek its comfort and everything that will appeal to their self‑love.
There is no reason to be astonished. The venial sins which they commit continuously bind the gifts of the Holy Ghost, and it is no wonder that the effects of the gifts are not evident in them. It is true that these gifts grow together with charity habitually and in their physical being, but they do not grow actually and in the perfection which corresponds to the fervor of charity and increases merit in us, because venial sins, being opposed to the fervor of charity, impede the operation of the gifts of the Holy Ghost.
If these religious would strive for purity of heart, the fervor of charity would increase in them more and more and the gifts of the Holy Ghost shine forth in their conduct; but this will never be very apparent in them, living as they do without recollection, without attention to their interior life, letting themselves be led and guided by their inclinations, and avoiding only the more grave sins while being careless about little things.
The Effects of Venial Sin
Venial sin has four effects in this life and certain effects in the life to come.
1) It deprives us of many actual graces which God would otherwise have given us. This privation sometimes results in our falling into a temptation which we could have avoided by means of that actual grace of which we were deprived. At other times it may result in the loss of a new advance in the spiritual life. It likewise results in a lessening of the degree of glory which we would have attained through resistance to that temptation or through the increase in grace. Only in the light of eternity—and then there is no remedy—shall we realize what we have lost as a result of deliberate venial sins.
2) It lessens the fervor of charity and one's generosity in the service of God. This fervor and generosity presuppose a sincere desire for perfection and a constant striving for it, which are totally incompatible with voluntary venial sin, because the latter implies a rejection of that lofty ideal and a deliberate halt in the struggle for greater holiness.
3) It increases the difficulties in the exercise of virtue. This is a result of the two previous effects. Deprived of many actual graces which are necessary to keep us on the path of the good and having lost a good part of its fervour and generosity in the service of God, the soul is gradually weakened and loses more and more of its spiritual energy. Virtue appears to be more difficult, the effort required for growing in holiness becomes more and more demanding, the experience of past failures for which we ourselves are responsible disheartens the soul, and while the world attracts the soul with its seductions and the devil intensifies his attacks, the soul ultimately abandons the path of perfection and perhaps gives itself without resistance to sin.
4) It predisposes for mortal sin. This is clearly testified in Scripture when it is stated that he who wastes the little he has is gradually stripped bare. Experience confirms this proof. The soul seldom falls directly and immediately, however violent the attack of its enemies. Usually, the ultimate fall of a soul has been prepared little by little. The soul has gradually lost ground to the enemy, it has been losing its strength through voluntary imprudence in matters which it considered of little importance, it has been losing the divine inspirations, and little by little it has lowered its defenses until the moment arrives in which the enemy, in one furious assault, conquers the city.
5) The reason for the sufferings of purgatory is the punishment and purification of the soul. Every sin, in addition to the fault, carries with it the guilt of punishment which must be satisfied in this life or in the next. The punishment due to mortal sins already pardoned and that of venial sins, whether pardoned or not, not satisfied in this life is satisfied in purgatory. God cannot renounce His justice, and the soul must pay its debt completely before it can be admitted to beatific joy. And the pains which the soul will have to suffer in purgatory for those faults which on earth it considered light and of small importance surpass the greatest pains which one could suffer in this world. St. Thomas says this expressly when he teaches that there are two types of pain in purgatory: the one consists in the delay in the reception of the beatific vision and the other which consists in the torment caused by a material fire. And the smallest amount of either one surpasses the greatest suffering in this world.
6) The increases of grace, of which the soul is deprived in this life because of venial sins, will have a repercussion in eternity. The soul in heaven will have a lesser glory than it could have attained had it been more faithful to grace in this life. For that reason, for all eternity it will be giving less glory to God than it could have. The degree of glory is in direct relation to the degree of grace attained in this life,
Combating Venial Sin
It is above all necessary to conceive a great horror for venial sin. We shall never begin to make serious progress in our sanctification until we have done this. To this end, it will be of great help to consider often what we have said concerning its malice and consequences. We must return again and again to the battle against venial sin and never give it up even for an instant.
Actually, because of pauses and vacations in the life of fervor and of constant vigilance, one readily cultivates indolence and cowardice. It is necessary to be faithful to the examination of conscience, both general and particular; to increase one's spirit of sacrifice; to be faithful to the practice of prayer; to safeguard external and internal recollection to the extent that the duties of our state permit; and to remember the example of the saints, who would rather have died than commit a deliberate venial sin. When we have succeeded in cultivating this disposition in our soul in a permanent and habitual manner, when we are disposed with promptness and facility to practice any sacrifice necessary to avoid deliberate venial sin, we shall arrive at the second negative degree of piety, which consists in flight from venial sin. It is not an easy task. If in the first degree—the avoidance of mortal sin—such a great struggle was necessary, what can we say about the avoidance and flight from venial sin? But however difficult it may be, it is possible to approach that ideal by means of a constant struggle and humble prayer until one has reached the same status as that which was achieved by the saints.
Although this matter is greatly disputed among theologians, we believe that moral imperfection is something distinct from venial sin. An act which is good in itself does not cease to be good even though it could have been better. Venial sin, on the other hand, is something intrinsically evil, however light an evil may be. There is a great difference between the two. In theory the distinction between venial sin and imperfection seems very clear. In practice, nevertheless, the fully voluntary imperfection has harmful effects on the spiritual life and is of itself sufficient to impede the flight of the soul to sanctity. St. John of the Cross treats of this matter with great clarity when he distinguishes between venial sin and imperfection:
But all the other voluntary desires, whether they be of mortal sin, which are the gravest, or of venial sin, which are less grave, or whether they be only of imperfections, which are the least grave of all, must be driven away every one, and the soul must be free from them all, howsoever slight they be, if it is to come to this complete union; and the reason is that the state of this divine union consists in the soul's total transformation, according to the will, in the will of God, so that there may be naught in the soul that is contrary to the will of God, but that, in all and through all, its movement may be that of the will of God alone. . . . For if this soul desired any imperfection that God wills not, there would not be made one will of God, since the soul would have a will for that which God has not.
It is clear, then, that for the soul to come to unite itself perfectly with God through love and will, it must first be free from all desire of the will, howsoever slight. That is, it must not intentionally and knowingly consent with the will to imperfections, and it must have power and liberty to be able not so to consent intentionally. I say "knowingly," because, unintentionally and unknowingly, or without having the power to do otherwise, it may well fall into imperfections and venial sins, and into the natural desires whereof we have spoken; for of such sins as these which are not voluntary but surreptitious it is written that the just man shall fall seven times in the day and shall rise up again.
But of the voluntary desires, which, though they be for very small things, are, as I have said, intentional venial sins, any one that is not conquered suffices to impede union. I mean, if this habit be not mortified; for sometimes certain acts of different desires have not as much power when the habits are mortified. Still, the soul will attain to the stage of not having even these, for they likewise proceed from a habit of imperfection. But some habits of voluntary imperfections, which are never completely conquered, prevent not only the attainment of divine union but also progress in perfection.
These habitual imperfections are, for example a common custom of much speaking, or some slight attachment which we never quite wish to conquer—such as that to a person, a garment, a book, a cell, a particular kind of food, tittle‑tattle, fancies for tasting, knowing or hearing certain things, and suchlike. A single one of these imperfections, if the soul has become attached and habituated to it, is of a great harm to growth and progress in virtue as though one were to fall daily into a great number of other imperfections and casual venial sins which do not proceed from a habitual indulgence in some harmful attachment; these latter imperfections will not hinder the soul so much as will its attachment to something. For as long as it has this, there is no possibility that it will make progress in perfection, even though the imperfection be extremely slight. For it comes to the same thing whether a bird be held by a slender cord or by a stout one since, even if it be slender, the bird will be as well held as though it were stout, for so long as it breaks it not and flies not away. It is true that the slender one is the easier to break; still, easy though it be, the bird will not fly away if it be not broken. And thus the soul that has attachment to anything, however much virtue it possesses, will not attain to the liberty of divine union.
As can be seen, St. John of the Cross points out the basic reason why it is necessary to renounce absolutely all voluntary imperfections. At the same time, he emphatically distinguishes between voluntary imperfections and those which proceed from pure weakness or inadvertence. He does well to distinguish between an isolated act, though deliberately imperfect, and the deeply rooted habit of voluntary imperfection. It is the latter which impedes perfect union with God.
Lack of Progress
The magnificent doctrine of this great mystic finds confirmation in the Thomistic doctrine on the increase of habits. According to St. Thomas, charity and all the other infused habits increase only by a more intense act which flows from an actual grace itself more intense than the habit. Otherwise, a more intense act of any virtue would be impossible, because one cannot give what he does not have. It follows from this that prayer is of extreme importance in this regard, because the only way in which we can obtain actual grace is by impetration, since it does not fall under merit in the proper sense of the word. Now imperfection is by its very nature a remiss act or the voluntary negation of a more intense act. Consequently, it is impossible to proceed in perfection if one does not renounce habitual voluntary imperfections.
This is the reason why in practice so many potential saints are frustrated and why there are so few true saints. There are many souls who live habitually in the grace of God, who never commit mortal sins and even exert every effort to avoid venial sins. Nevertheless, they are paralyzed in the spiritual life, and they remain for many years in the same imperfections or even grow in imperfections. How can we explain this phenomenon? The answer is that they have not endeavored to root out their voluntary imperfections; they have not tried to break that slender cord which keeps them tied to the earth and prevents them from rising in flight to the heights. With what accents of pity and sadness St. John of the Cross laments this situation:
It is sad to see certain souls in this plight; like rich vessels, they are laden with wealth and good works and spiritual exercises, and with the virtues and the favors that God grants them; and yet, because they have not the resolution to break with some whim or attachment or affection (which all come to the same thing), they never make progress or reach the port of perfection, though they would need to do no more than make one good flight and thus to snap that cord of desire right off, or to rid themselves of that sucking‑fish of desire which clings to them.
It is greatly to be lamented that, when God has granted them strength to break other and stouter cords—namely, affections for sins and vanities—they should fail to attain to such blessing because they have not shaken off some childish thing which God had bidden them conquer for love of Him, and which is nothing more than a thread or a hair. And, what is worse, not only do they make no progress, but because of this attachment they fall back, lose that which they have gained, and retrace that part of the road along which they have traveled at the cost of so much time and labor; for it is well‑known that, on this road, not to go forward is to turn back, and not to be gaining is to be losing. This Our Lord desired to teach us when He said: "He that is not with me is against me; and he that gathereth not with me scattereth." He that takes not the trouble to repair the vessel, however slight be the crack in it, is likely to spill all the liquid that is within it. The Preacher taught us this clearly when He said: "He that contemneth small things shall fall by little and little." For, as He Himself says, a great fire cometh from a single spark. And thus one imperfection is sufficient to lead to another; and these lead to yet more; wherefore you will hardly ever see a soul that is negligent in conquering one desire, and that has not many more arising from the same weakness and imperfection that this desire causes. In this way they are continually falling; we have seen many persons to whom God has been granting the favor of leading them a long way, into a state of great detachment and liberty, yet who, merely through beginning to indulge some slight attachment, under the pretext of doing good, or in the guise of conversation and friendship, often lose their spirituality and desire for God and holy solitude, fall from the joy and wholehearted devotion which they had in their spiritual exercises, and cease not until they have lost everything; and this because they broke not with that beginning of sensual desire and pleasure and kept not themselves in solitude for God.
It is therefore absolutely necessary to wage an unceasing battle against our voluntary imperfections if we wish to arrive at perfect union with God. The soul must use all its efforts and all its energies to make them disappear. It must tend always toward the more perfect and try to do all things with the greatest possible intensity. Naturally, this greater intensity should not be considered as a physical or organic intensity, as if it were necessary to keep one's nervous system in a state of constant tension or to make an act of love of God accompanied by organic or psychic intensity. We are referring here simply to the perfection of one's motives which lead one to act: doing all things with the greatest possible purity of intention, with the greatest possible desire of glorifying God, with the ardent desire that God's action invade or dominate us completely, that the Holy Ghost take complete control of our souls and do with us as He wishes in time and in eternity, without taking any account of our own tastes or desires. It consists simply in an ever more perfect and docile abandonment to the will of God until we are led by Him without the least resistance. And this will not occur before the total death of our human egoism and our full transformation in Christ, which will enable us to say with St. Paul: "It is now no longer 1 that live, but Christ liveth in me" (Gal. 2:20).
It is evident that this profound transformation of our being and this complete death of our ego is an enterprise that surpasses human power, even assisted by ordinary grace. As long as man takes the initiative in his Christian life through the simple practice of the acquired virtues in a human mode, it is impossible to attain that profound purification of our innermost being. It is necessary that the Holy Ghost Himself effect this transformation in its double aspect of the negative and the positive. St. John of the Cross expressly states this, and the obvious conclusion which follows is that sanctity is impossible outside the mystical life.
 Theology of Christian Perfection, Antonio Royo Marin, O.P., Part III, ch. 2, pgs. 184 - 192
 And grieve not the holy Spirit of God: whereby you are sealed unto the day of redemption (Eph. 4, 30).
 A workman that is a drunkard shall not be rich: and he that contemnes small things, shall fall by little and little (Ecclesiasticus 19, 1).