15 “Forgiveness of sins.” You have [this article of] the Creed perfectly in you when you receive Baptism. Let none say, “I have done this or that sin: perchance that is not forgiven me.” What have you done? How great a sin have you done? Name any heinous thing you have committed, heavy, horrible, which you shudder even to think of: have done what you will: have you killed Christ? There is not than that deed any worse, because also than Christ there is nothing better.
What a dreadful thing is it to kill Christ! Yet the Jews killed Him, and many afterwards believed on Him and drank His blood: they are forgiven the sin which they committed. When you have been baptized, hold fast a good life in the commandments of God, that you may guard your Baptism even unto the end. I do not tell you that you will live here without sin; but they are venial, without which this life is not. For the sake of all sins was Baptism provided; for the sake of light sins, without which we cannot be, was prayer provided. What has the Prayer?
“Forgive us our debts, as we also forgive our debtors.” Once for all we have washing in Baptism, every day we have washing in prayer. Only, do not commit those things for which you must needs be separated from Christ's body: which be far from you! For those whom you have seen doing penance, have committed heinous things, either adulteries or some enormous crimes: for these they do penance. Because if theirs had been light sins, to blot out these daily prayer would suffice.
Source: On the Creed: A Sermon to Catechumens (New Advent)