16 In three ways then are sins remitted in the Church; by Baptism, by prayer, by the greater humility of penance; yet God does not remit sins but to the baptized. The very sins which He remits first, He remits not but to the baptized. When? When they are baptized. The sins which are after remitted upon prayer, upon penance, to whom He remits, it is to the baptized that He remits. For how can they say, “Our Father,” who are not yet born sons? The Catechumens, so long as they be such, have upon them all their sins. If Catechumens, how much more Pagans? How much more heretics? But to heretics we do not change their baptism. Why? Because they have baptism in the same way as a deserter has the soldier's mark: just so these also have Baptism; they have it, but to be condemned thereby, not crowned. And yet if the deserter himself, being amended, begin to do duty as a soldier, does any man dare to change his mark?
Source: On the Creed: A Sermon to Catechumens (New Advent)