In the early Church, the practice of the sacrament of confession was not a very common affair. While there is certainly testimony to the confession of light sins in the sacrament of confession, it was most associated with severe sins that demanded a canonical and public penance. St. Augustine frequently connects the forgiveness of light sins to the prayer "forgive us our sins, as we forgive those who sin against us":
Therefore baptism is sealed with the seal of Christ, that is, when you are dipped in the water, and as it were passed through the red sea. Your sins are your enemies; they follow, but only unto the sea. When you enter it, you will escape them, they will be destroyed, as the water covered the Egyptians while the Israelites escaped through dry land. And what does the Scripture say? Not one of them remained. You have sinned many sins, you have sinned few sins; you have sinned great sins, you have sinned small sins. What of it, when not one of them remained? But since you are going to live in this world, where no one lives without sin, therefore the forgiveness of sins is not only in the washing of holy baptism, but also in the Lord's prayer, a daily prayer, which you will receive after eight days. In it you will find your daily baptism, as it were, so that you give thanks to God, who gave this gift to his Church, which we confess in the creed, so that when we say “holy Church,” we add “forgiveness of sins.” (Sermon 213)
When ye have been baptized, hold fast a good life in the commandments of God, that ye may guard your Baptism even unto the end. I do not tell you that ye 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 hath 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 ye must needs be separated from Christ’s body: which be far from you! For those whom ye 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. (The creed: A Sermon To The Cathechumens)
The canonical penance, like baptism, could only be made once. This did not necessarily imply despair over the salvation of the salvation of those men who fell back into sin while undergoing penance or afterward, though it might suggest some doubt about the certainty of the genuineness of repentance:
There are some men whose wickedness goes so far that, after having undergone penance, and been reconciled to the altar, commit the same sins again, or even worse sins. And yet God, who makes his sun rise even over such persons, does not grant any less than before the gift of life and salvation. And although they are given no opportunity for penance in the Church, God does not forget his patience toward them.
If one of them says to us: “either give me again an opportunity for penance or proclaim as beyond hope, so that I may do whatever I want, so far as my resources and human laws allow me, having intercourse with prostitutes and abandoning myself to all kinds of lust that are condemned in the eyes of God though praised by most men. Or if you call me away from this iniquity, tell me if for the future life it is of any value for me to despise the blandishments of illicit pleasure, for me to deny the incitements of lust, if in order to chastise my body I deny myself even many things licit and granted to me, if I torment myself even more than before in penance, if I groan with greater sorrow, if I weep more abundantly, if I live better, if I give more bountifully to the poor, if I burn more ardently with the charity that covers a multitude of sins?” who of us is so foolish as to say to that man, “none of that will benefit you in the future; go, enjoy at least the sweetness of this life”? May God keep us from such a monstrous sacrilege and madness!
Although, therefore, for reasons of prudence and for the sake of the salvation of souls the Church's discipline provides opportunity for humbling oneself in penance only once, lest the medicine be seen as cheap, and thus less useful for the sick, seeing as the less it is despised, the more salvific it will be, who will dare to say to God: “Why do you again forgive this man who after having once embraced penance again bound himself in the snares of sin?” (Epistle 153)
Although Augustine generally distinguishes grave sins, for which one must be separated from the Body of Christ, and undergo a period of penance, from light sins, forgiven through the Lord's Prayer–"Forgive us, as we forgive others"–this distinction does not consistently line up with the distinction mortal-venial.
17. … Whenever that carnal or animal sense introduces into this purpose of the mind which is conversant about things temporal and corporeal, with a view to the offices of a man’s actions, by the living force of reason, some inducement to enjoy itself, that is, to enjoy itself as if it were some private good of its own, not as the public and common, which is the unchangeable, good; then, as it were, the serpent discourses with the woman. And to consent to this allurement, is to eat of the forbidden tree. But if that consent is satisfied by the pleasure of thought alone, but the members are so restrained by the authority of higher counsel that they are not yielded as instruments of unrighteousness unto sin; this, I think, is to be considered as if the woman alone should have eaten the forbidden food.
But if, in this consent to use wickedly the things which are perceived through the senses of the body, any sin at all is so determined upon, so that if possible it is also fulfilled by the body, then that woman must be understood to have given the unlawful food to her husband to eat it together with her. For it is not possible for the mind to determine that a sin is not only to be thought of with pleasure, but also to be effectually committed, unless also that intention of the mind yields, and serves the bad action, with which rests the chief power of applying the members to an outward act, or of restraining them from one.
18. And yet, certainly, when the mind is pleased in thought alone with unlawful things, while not indeed determining that they are to be done, but yet holding and pondering gladly things which ought to have been rejected the very moment they touched the mind, it cannot be denied to be a sin, but far less than if it were also determined to accomplished it in outward act. And therefore pardon must be sought for such thoughts too, and the breast must be smitten, and it must be said, “Forgive us our debts;” and what follows must be done, and must be joined in our prayer, “As we also forgive our debtors.” For it is not as it was with those two first human beings–in that case, each one bore his own person, and so, if the woman alone had eaten the forbidden food, she indeed alone would have been smitten with the punishment of death; we cannot say this in the case of a single human being now, that if the thought, remaining alone, be gladly fed with unlawful pleasures, from which it ought to turn away directly, while yet there is no determination that the bad actions are to be done, but only that they are retained with pleasure in remembrance, the woman as it were can be condemned without the man. Far be it from us to believe this. For here is one person, one human being, and he as a whole will be condemned, unless those things which, as lacking the will to do, and yet having the will to please the mind with them, are perceived to be sins of thought alone, are pardoned through the grace of the Mediator. (On the Trinity XII, ch. 12, emphasis added.)
Inasmuch as the Church's belief is manifested in her practice, the discipline of penance in the Church may support one or another interpretation of the common courses of a Christian's life.
While not having a strictly logical connection with it, the practice of having penance after baptism be a one-time only affair would harmonize with the idea that the fundamental orientation of people's lives cannot be expected to rapidly and frequently change, and in this sense the early practice of the Church would support with a certain interpretation of a fundamental option.