We Protestants believe that Christian doctrine was kept pure as long as the Apostles lived, hut after their deaths, errors crept in.

You err both in fact and in doctrine. In fact, for the Apostles complained of errors, not of the Church, but of individual professing Christians even in their own days. In doctrine, because you practically assert that Christ failed to preserve His Church; that the Holy Spirit did not remain with her; and that the gates of hell did prevail against her. In other words, your doctrine is that Christ could not do what He said He would do. No. Individuals in all ages have fallen into error in so far as they departed from the teachings of the Church. And in falling into error, they have fallen out of the Church, even as the Protestant Reformers themselves.

Radio Replies Volume 1 by Rev. Dr. Leslie Rumble MSC and Rev. Charles Mortimer Carty