What the Church has always taught

Regarding participating in Non-Catholic Worship as found in her Catechetical teachings.

Against this [First] Commandment all those sin who have not faith, hope and charity. Such sinners are very numerous, for they include all who fall into heresy, who reject what holy mother the Church proposes for our belief, who give credit to dreams, fortune-telling, and such illusions; those who, despairing of salvation, trust not in the goodness of God; and those who rely solely on wealth, or health and strength of body. But these matters are developed more at length in treatises on sins and vices. [1]

[The First Commandment] does prohibit and condemn all idolatry and worshipping of false Gods, art magic, divination, superstitions observations, and all wicked worshipping. And upon the contrary part it requires, that we believe in God and worship him. …Who be they, that break the first commandment of God by Infidelity? All heretics, idolaters, Turks and Jews, and all they that do not profess the Catholic faith both in heart, word and deed. [2]

How do men sin against this [First] commandment? By worshipping idols and false gods, by erring or doubting in faith, by superstition and witchcraft. How else? By communicating with infidels or heretics, by believing dreams, etc. How do you prove it a great sin to go to church with heretics? Because by so doing we outwardly deny our faith, and profess their false faith. What scripture have you against it? Out of Luke 17:23-24, where Christ forbids it, saying “And they shall say unto you, Lo! here is Christ, Lo, there Christ; go ye not, neither do you follow them.” What other proof have you? Out of Tit 3:10-11: “A man that is a heretic, after the first and second admonition, avoid, knowing that he that is such an one is subverted and sinneth.” [3]

…[W]e must believe without exception all such articles as by God and his Church are proposed to be believed: and he that voluntarily and obstinately disbelieveth any one of these articles, is no less void of true saving faith, than he that disbelieves them all: as St. James tells us with regard to practical duties (2:10), “Whosoever shall keep the whole law, yet offend in one point, is guilty of all.” Hence St. Paul (Gal 5:20), reckons heresies, that is, false religions, among those works of the flesh, of which he pronounces, “that they who do such things shall not inherit the kingdom of God,” and God himself (Isa 60:12), tells his Church; “The nation and kingdom that will not serve thee shall perish.” [4]

All communication in matters of religion with those separated from the Church of Christ… is a very great crime in the sight of God, and strictly forbidden by His holy law, as being intrinsically evil in its own nature. …[W]e are expressly forbidden all communication in their religion that is, in their false tenets and worship. …Wherefore, to all those arguments which may be brought from human, worldly, or interested motives, to induce us to join in or to partake of any religious duty with those of a false religion, though in appearance only, we ought to oppose this one,—”God has expressly forbidden it, therefore no human power can make it lawful.” [5]

How do we sin against the exterior worship of God? By neglecting to attend divine service, or by behaving irreverently when we are present. May we sin in any other way against the reverence due to God? Yes, we sin also against it by idolatry, superstition, witchcraft, sacrilege, and simony.   When do we sin by superstition? When we honor God or the Saints in a manner contrary to the doctrine or practice of the Church… [6]

What is the virtue of religion? The virtue of religion is a habit by which we render to God the worship which is due to him. Man, being composed of a body and soul, owes to God the homage of his whole being; hence the necessity of exterior as well as interior worship.   What are the sins opposed to the virtue of religion? They are irreligion, superstition, and unlawful worship.   …What is unlawful worship? Unlawful worship consists in worshiping God in a manner different from what he prescribes. Unlawful worship and superstition are artifices of the devil, by which he would disfigure religion, detach men from God, draw them to himself, and finally destroy them. [7]

If the mere attending other places of worship be wrong for a Catholic, how grievously sinful is it to take part in the worship, prayers, and other religious functions — to conform and act as if a member of such church! This is emphatically the sin of “communication in divine things” with those not of the household of the faith. The translators of the New Testament, published first at Rheims, say in a note: “That in matters of religion, in praying, hearing their sermons, presence at their service, partaking of their sacraments, and all other communicating with them in spiritual things, it is a great and damnable sin.”   …This will apply to Catholics of all places and times. Though many, either thoughtlessly or with indifference, act contrary to the divine commands, and the laws of the Church, and frequent other places of worship from curiosity, fashion, or for the sake of friends, let them feel assured that they scarcely ever can do this without sin, and that they will have to give an account to God for exposing their faith and giving scandal, as also for seeming to approve separated churches. [8]

What are the sins against faith? Infidelity, heresy, apostasy, indifference to faith, and wilfull doubt of any article of faith, all of which are mortal sins.    …How may we commit sin against the worship and adoration of God? By worshipping false gods or idols, or by giving to any creature whatsoever the  honor which is due to God alone, to do which is always a mortal sin.   May one commit sin against the adoration of God in any other way? Yes; by attending the false worship of any non-Catholic religious society, by sacrilege and irreverence, by simony, superstition, witchcraft, and spiritualism. [9]

What is to adore God? It is to acknowledge him, by inward and outward acts of worship, as our Creator and sovereign Lord.   What are the sins agains the adoration of God? Idolatry, sacrilege, irreverence, simony, witchcraft, sorcery, superstition, spiritism, and attendance at false worship.   …What is attendance at false worship? It is to assist at the religious worship of heretics. [10]

Which are the sins against the adoration of God? The sins against the adoration of God are: Superstition and irreligion.   …What is superstition? …There are four kinds of superstition, namely: idolatry, attendance at the false worship of the true God, divination, and superstitious practices.   …What is attendance at false worship? It is to assist at the religious worship of heretics. …”But the bishop, or parish priest has given me permission to play the organ, to sing, etc., in the Protestant church,” says another one. I answer: Neither any priest nor bishop, nay, not even the Pope, can give you permission to violate any of the commandments. “But I am well instructed in my religion,” says another; “I can see no harm in what I do in the Protestant church.” I answer: I doubt what you say. If you were well instructed, you would know that attendance at false worship is a mortal sin, and that this sin is still greater for him who plays, or sings, at it, or renders any other kind of service for it. And do you see no harm in committing a mortal sin? Do you see no harm in the great scandal you give to those Catholics who know of it, and to the Protestants, whom by your playing and singing, etc., you confirm in the belief that their religion is as good as the Catholic religion? [11]

How may the First Commandment be broken? The First Commandment may be broken by giving to a creature the honor which belongs to God alone; by false worship; and by attributing to a creature a perfection which belongs to God alone… “False worship”—that is, worshipping God not as He directs us by His Church, but in some ways pleasing to ourselves. For example, to sacrifice animals to God would now be false worship; to offer now any of the sacrifices commanded in the Old Law would be false worship… So, too, all those who leave the true Church to practice a religion of their own have a false worship, for they worship God not as He wishes, but as they wish. [12]

Sharing in non-Catholic worship sinful: No cause can justify a Catholic in actively sharing in non-Catholic acts of public worship, nor in Catholics doing anything that in the actual circumstances has truly the appearance of adhesion to non-Catholic beliefs, no matter how loyal to the faith he may be in his own heart.   …Catholic faith and worship alone true: The position in which our Catholic faith places us is this: We believe that this faith, and it alone, is the one true faith of Jesus Christ, the Son of God, and that the Catholic Church in communion with the See of Rome is His One True Catholic Church, and supplies us with the only genuine form of Christian worship. That is our position, or it is nothing. …To the Catholic it is a first principle—an absolute and unchangeable law of God’s own making, which is superior to him, and carries with it certain very definite duties, any shuffling about which would be not merely unprincipled, but a shamefaced denial of Christ before men. [13]

…In the First Commandment God enjoins upon us to worship Him, and forbids idolatry and every false form of worship. …He who does not worship God in the manner which He has revealed and which the Church prescribes, will ere long come to worship Him after a debased and foolish fashion. This is the false worship of God. [14]

What are the sins against Faith? The sins against Faith are all false religions, willful doubt, disbelief, or denial of any article of Faith, and also culpable ignorance of the doctrines of the Church. [15]

…[T]o assert that a person can be saved out of the true Church of Christ, is neither more nor less than to say that God is indifferent to any form of worship, and that the worship paid Him by those who, for example, deny that there are three Persons in God, or that God became man, is equally as pleasing to Him as that paid by those who acknowledge Jesus Christ as their Redeemer, and who firmly believe in everything that Jesus Christ has taught; that He is equally pleased with those who distort, falsify, and refuse to believe His word, as with those who religiously observe all His words, and believe them. Can anything be imagined more absurd, more revolting, than to say that God, whom we serve, is indifferent to truth or error! Yet this is the principle of all those who say that every form of worship is good, and that one can be saved in whatever religion he professes. [16]

Who has convoked or called us into the Church of Jesus Christ? We have been called into the Church of Jesus Christ by a special grace of God, to the end, that by the light of faith and the observance of the divine law, we may render Him the worship due to Him, and attain eternal life.    …What is forbidden by the First Commandment? The First Commandment forbids idolatry, superstition, sacrilege, heresy, and every other sin against religion.    … What is superstition? Superstition is any devotion that is contrary to the teaching and practice of the Church; as also the ascribing to any action or any thing whatever a supernatural virtue which it does not possess.   …What else does the First Commandment forbid? The First Commandment also forbids all dealings with the devil, and all association with anti-Christian sects. [17]

What are the sins against faith? The sins against faith are all false religions, wilful doubt, disbelief, or denial of any article of faith, and also culpable ignorance of the doctrines of the Church.   …How do we expose ourselves to the danger of losing our faith? We expose ourselves to the danger of losing our faith by neglecting our spiritual duties, reading bad books, going to non-Catholic schools, and taking part in the services or prayers of a false religion. …It is a mortal sin for Catholics to attend non-Catholic places of worship for the purpose of taking part in the services, because they know that all religions except their own are false. [18]

What are the sins opposed to the virtue of religion? They are of two kinds: those of excess, which come under the name of superstition; and those of defect, which are comprised under the name of irreligion.   What is meant by superstition? By superstition is understood that complexity of sins which consists in paying worship to God such as cannot be pleasing to Him; or to pay to things other than God the worship that belongs to Him alone. [19]

What are we told to do by the First Commandment? We are commanded to adore God alone.   …What is forbidden by this Commandment? (a) This Commandment forbids us to adore false gods. (b) This Commandment forbids us to give to anyone or anything the adoration and service that belong to God. (c) This Commandment forbids us to adore God by false worship. [20]

How can a Catholic best safeguard his faith? A Catholic can best safeguard his faith by making frequent acts of faith, by praying for a strong faith, by studying his religion very earnestly, by living a good life, by good reading, by refusing to associate with the enemies of the Church, and by not reading books and papers opposed to the Church and her teaching.   …How does a Catholic sin against faith? A Catholic sins against faith by apostasy, heresy, indifferentism, and by taking part in non-Catholic worship. [21]

The First Commandment enjoins the worship of God by Faith, Hope, Charity and True Religion. It commands us to believe firmly in God and in His word, to learn what He has taught and to profess our belief. It bids us also to trust or hope in God, to love Him, and to adore Him by prayer and sacrifice. …Therefore, all forms of false religion – all those not established by God Himself – are forbidden. It is obviously wrong to maintain that religions established by men, with no divine authority or mission, are equal to that which was established by God. One religion is not as good as another. …Willfully to expose oneself to the danger of losing the Faith would be a grievous sin. We expose ourselves to the danger of losing our Faith by: a) Committing sin, especially mortal sin… b) Failing to pray… c) Failing to study our faith… d) Neglecting our spiritual duties… e) Reading bad books… f) Going to non-Catholic schools… g) Taking part in the services or prayers of a false religion: Protestants usually believe that all Christian religions are as good as one another, that the denominations are merely various branches of Christ’s Church. Hence, they usually have no objection to attending Catholic services. But Catholics believe that their own Church is the only one founded by God Himself and that all others are false. Hence, it is illogical of Catholics to attend services held by ministers of false religions. [22]

What are the sins against Faith? The sins against Faith are all false religions, wilful doubt, disbelief, or denial of any article of Faith and also culpable ignorance of the doctrines of the Church.   How do we expose ourselves to the danger of losing our Faith? We expose ourselves to the danger of losing our Faith by neglecting our spiritual duties, reading bad books, going to non-Catholic schools, and taking part in the services or prayers of a false religion. Suppose you had all your treasures in one room, or one steel safe: jewels, letters, all the money you had to live on the rest of your life-what care you would take not to lose the key! Our Faith is our most precious possession-the key to our happiness in this life and next. Yet our Faith can be lost! [23]

HOW DOES A CATHOLIC SIN against faith? A Catholic sins against faith by infidelity, apostasy, heresy, indifferentism, and by taking part in non-Catholic worship.   …WHY DOES A CATHOLIC SIN against faith by taking part in non-Catholic worship? A Catholic sins against faith by taking part in non-Catholic worship, because he thus professes belief in a religion he knows is not the true one. 1. It is wrong to be present at non-Catholic services even when we do not participate in them, because such services are intended to honor God in a manner He does not wish to be honored in. If God instituted a Church of His own, He must wish to be honored in the ways of that Church. [24]

1 The Roman Catechism (1566) Part 3 (The Decalogue). The First Commandment. Sins Against This Commandment.
2 A Catechism or Christian Doctrine (1567) Ch. 3. Of the First Commandment.
3 The Douay Catechism (1649) Q. 384 -388.
4 Challoner’s Catholic Doctrine (1775) Ch. 1. Sec. 2
5 Hay’s Catechism (1781) Appx. On Communicating in Religion With Those Who are Separated from the Church of Christ. Q. 9-11, 13, 16 
6 Deharbe’s Catechism (1848) Ch. 2. The First Commandment of God. Sec. 1. Q. 23-24, 26.
7 Catechism of Perseverance (1849) Ch. 26. First Commandment.
8 Faith the Victory (1865) Ch. 9 (The Constitution of the Church).
9 Familiar Explanation of Christian Doctrine (1874) Part 1. Lesson 7.; Part 3. Lesson 2.
10 Catechism of Christian Doctrine (1877) Vol. III. Ch. 3. The First Commandment. Q. 3-4, 14-16.3.
11 God the Teacher of Mankind (1881) The Ten Commandments. The First Commandment. Q. 4-5, 7.
12 Baltimore Catechism (1885) Vol. IV. Q. 318.
13 Letters on Christian Doctrine (1897) The First Commandment. No. 3 (Faith and Its Duties). … No. 4 (Duties of Faith in Relation to Non-Catholics).
14 The Catechism Explained (1899) Ch. 10. The First Commandment of God.
15 The Penny Catechism (1905) Ch. 3. The Ten Commandments. The First Commandment. Q. 177.
16 Power’s Catechism (1905) Vol. I. Ch. 34 (On the True Church). No. 8.
17 Catechism of Pope St. Pius X  (1908) The Ninth Article of the Creed. The Church in General. Q. 4.; On the Commandments of God and of the Church. The Commandments of God in General. The First Commandment. 8, 12, 14-15.
18 The Catechism Simply Explained (1922) Ch. 5 (Charity). The Commandments of God. The First Commandment. Q. 177-178.
19 Catechism of the Summa Theologica (1922) Sec. 2, Ch. 31. (A).
20 Malloy’s Catechism (1927) Ch. 5. Sec. 1. Q. 5-6, 9-10.
21 Baltimore Catechism – Confraternity Ed. (1941) Vol. II. Lesson 16.
22 This Is the Faith (1951) Chapter 8 (1). The First Commandment.
23 Catechism At Early Mass (1957) Q. 176-178.
24 My Catholic Faith (1963) Part 2. Sec. 96.