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 one not a Catholic doubts whether the church to which he belongs is the true Church, he must settle his doubt, seek the true Church, and enter it; for if he continues to live in doubt, he becomes like the one who knows the true Church and is deterred by worldly considerations from entering it.
In like manner one who, doubting, fears to examine the religion he professes lest he should discover its falsity and be convinced of the truth of the Catholic faith, cannot be saved.
Suppose, however, that there is a non-Catholic who firmly believes that the church to which he belongs is the true Church, and who has never—even in the past—had the slightest doubt of that fact—what will become of him?
If he was validly baptized and never committed a mortal sin, he will be saved; because, believing himself a member of the true Church, he was doing all he could to serve God according to his knowledge and the dictates of his conscience. But if ever he committed a mortal sin, his salvation would be very much more difficult. A mortal sin once committed remains on the soul till it is forgiven. Now, how could his mortal sin be forgiven? Not in the Sacrament of Penance, for the Protestant does not go to confession; and if he does, his minister—not being a true priest—has no power to forgive sins. Does he know that without confession it requires an act of perfect contrition to blot out mortal sin, and can he easily make such an act? What we call contrition is often only imperfect contrition—that is, sorrow for our sins because we fear their punishment in Hell or dread the loss of Heaven.
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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.