The doctrines of the Immaculate Conception and the Infallibility of the Pope were not believed before 1854 and 1870 respectively, yet had to be believed after those dates.

The Catholic Answers

Both doctrines were believed in so far as Catholics believed in the revelation given by Christ, which contained thesendoctrines implicitly. When the Church denned them she merely made explicit and of faith what had been hitherto implicit She gave, not a new truth, but simply made these matters clear by defining these doctrines to be part of the revelation brought us by Christ. The Church is here for that. Indeed, of what use is a teaching Church if she does not teach? All doubts concerning the correct interpretation of the original Christian doctrine on these two subjects were cleared away by these definitions, and to-day the 400 million Catholics in the world know the truth and accept it without hesitation.

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