The Enchiridion by St. Augustine
Chapter 65.--God Pardons Sins, But on Condition of Penitence, Certain Times for Which Have Been Fixed by the Law of the Church.
But even crimes themselves, however great, may be remitted in the Holy Church; and the mercy of God is never to be despaired of by men who truly repent, each according to the measure of his sin. And in the act of repentance, where a crime has been committed of such a nature as to cut off the sinner from the body of Christ, we are not to take account so much of the measure of time as of the measure of sorrow; for a broken and a contrite heart God doth not despise. But as the grief of one heart is frequently hid from another, and is not made known to others by words or other signs, when it is manifest to Him of whom it is said, |My groaning is not hid from Thee,| those who govern the Church have rightly appointed times of penitence, that the Church in which the sins are remitted may be satisfied; and outside the Church sins are not remitted. For the Church alone has received the pledge of the Holy Spirit, without which there is no remission of sins -- such, at least, as brings the pardoned to eternal life.