That would depend upon their convictions. We do not judge Protestants on principles they do not hold. If they think one religion as good as any other, I suppose they could attend almost any religion with a good conscience. But if an Anglican, for example, thought his to be the only true Church, and that all others were wrong, he wTould sin by attending other forms of worship. Objectively, of course, a Protestant does not sin by attending Catholic services. One who has the wrong religion may attend services of the right religion. But he who has the right religion certainly cannot attend the services of a wrong religion. Catholics may not assist at any but Catholic services.
Radio Replies Volume 1 by Rev. Dr. Leslie Rumble MSC and Rev. Charles Mortimer Carty