121. Q. Are all bound to belong to the Church?
A. All are bound to belong to the Church, and he who knows the Church to be the true Church and remains out of it, cannot be saved.
Anyone who knows the Catholic religion to be the true religion and will not embrace it cannot enter into Heaven…
If, then, we found a Protestant who never committed a mortal sin after Baptism, and who never had the slightest doubt about the truth of his religion, that person would be saved; because, being baptized, he is a member of the Church, and being free from mortal sin he is a friend of God and could not in justice be condemned to Hell. Such a person would attend Mass and receive the Sacraments if he knew the Catholic Church to be the only true Church.
I am giving you an example, however, that is rarely found, except in the case of infants or very small children baptized in Protestant sects. …
I said I gave you an example that can scarcely be found, namely, of a person not a Catholic, who really never doubted the truth of his religion, and who, moreover, never committed during his whole life a mortal sin. There are so few such persons that we can practically say for all those who are not visibly members of the Catholic Church, believing its doctrines, receiving its Sacraments, and being governed by its visible head, our Holy Father, the Pope, salvation is an extremely difficult matter.
I do not speak here of pagans who have never heard of Our Lord or His holy religion, but of those outside the Church who claim to be good Christians without being members of the Catholic Church.