For very good reasons. Firstly, loyalty to Christ forbids our sanctioning in any way a false form of religion, and Protestantism is a corruption of Christ’s religion. If one may attend any religious services, irrespective of creed, then a Christian could assist at pagan rites. There must be a limit somewhere, and the Catholic Church says that those limits exclude any false form of religion, even though it be an adulterated form of Christianity. The presence of a Catholic at Protestant services is a silent approval of the error that one religion is as good as another. St. Paul says, “A man that is a heretic avoid.” Tit. III., 10. St. John says, “If any man come to you and bring not this doctrine, receive him not into the house, nor say to him: ‘God speed you.'” 2 Jn. V., 10. The law of the Church, too, protects the faith of Catholics. If they attend Protestant services, there is always a danger that they will participate actively in a shamefaced way, and also a danger of their drifting into indifferentism and weakening in their own faith. Their presence, also, can be a cause of scandal to other Catholics who may begin to think that it is right for them also to attend at non-Catholic Churches. Nor is such attendance a kindness to Protestants. The abstention of Catholics from their services is a lesson of the utmost importance to them. Our attendance would sanction to a certain extent their idea that their religion also is as good as our own. But our absence from their Churches gives them food for thought. An Anglican might say, “Well, I have seen Methodists, Presbyterians, Congregationalists, and people of many other religions at our services; but I have never yet seen a Catholic associated with us.” And the fact that the vast Catholic Church denies their claims has led many a man from the chaos of the different Protestant Churches to the true religion.
Radio Replies Volume 1 by Rev. Dr. Leslie Rumble MSC and Rev. Charles Mortimer Carty