What is attendance at false worship - Fr. Michael Muller
To worship God according to a rite contrary to all precepts of the Gospel is a false and unlawful worship of God. Hence it would be a grievous sin for a Catholic to worship God according tot he ceremonial laws of the Jews, for though they were prescribed by God for the Jews before the coming of Christ, yet they were abolished by Christ in the new law.
It is also a false and unlawful worship of God to adopt a new religion in opposition to the doctrine of the true Church of Jesus Christ, the Roman Catholic Church, and assist at the religious worship of such a false religion. Hence, even if a Catholic despises in his heart such a false religion and worship, it is unlawful for him to play the organ, or to sing, or to discharge the office of sacristan, in Jewish or Protestant temples during their false worship, or to compose hymns or music for the same, or to ring the bell for calling the people together, or to contribute money towards the erection of temples for false worship, or to call a Protestant minister for the performance of some religious rites, as, for instance, the rites of marriage or baptism, or funeral, etc.; or to take Protestant Children, or accompany grown persons, to Protestant Sunday-schools or church and stay with them during their religious worship. Any such act is strictly forbidden by the law of God and of the Church, because it is a real communication and formal co-operation in a false worship, and a real approval of it.
Pope Paul IV, wrote to the Catholics in England: “We are forced to admonish and to conjure you, that on no account are you to go to churches of heretics, or hear their sermons or join in their rites, lest you incur the wrath of God; for it is not lawful for you to do such things without dishonoring God and hurting your own souls.”
Here one may say: The reason why I play the organ, or sing, etc., in a Protestant church, is because I get a pay which enables me to support my family. I answer: What you do is a grievous violation of the first commandment. It is never allowed to commit a mortal sin in order to acquire the means of support. Alas! that there are so many people who make a living by unlawful means ! "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 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?
Taken From - Teacher of Mankind, Pg. 331 -332.