From catholicity and covenant today:
In the order of the Creed, after the mention of the Holy Church is placed the remission of sins. For it is by this that the Church on earth stands: it is through this that what had been lost, and was found, is saved from being lost again. For, setting aside the grace of baptism, which is given as an antidote to original sin, so that what our birth imposes upon us, our new birth relieves us from (this grace, however, takes away all the actual sins also that have been committed in thought, word, and deed): setting aside, then, this great act of favor, whence commences man's restoration, and in which all our guilt, both original and actual, is washed away, the rest of our life from the time that we have the use of reason provides constant occasion for the remission of sins, however great may be our advance in righteousness. For the sons of God, as long as they live in this body of death, are in conflict with death. And although it is truly said of them, "As many as are led by the Spirit of God, they are the sons of God",yet they are led by the Spirit of God, and as the sons of God advance towards God under this drawback, that they are led also by their own spirit, weighted as it is by the corruptible body; and that, as the sons of men, under the influence of human affections, they fall back to their old level, and so sin ...
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 does 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 You",those who govern the Church have rightly appointed times of penitence, that the Church in which the sins are remitted may be satisfied.
From St. Augustine's Enchiridion (64-65).