For see yet again: "If we say," he says, "that we have not sinned, we make Him a liar, and His word is not in us." All the more fully: "Little children, these things have I written to you, lest ye sin; and if ye shall have sinned, an Advocate we have with God the Father, Jesus Christ the righteous; and, He is the propitiation for our sins.
After this we also entreat for our sins, saying, "And forgive us our debts, as we also forgive our debtors." After the supply of food, pardon of sin is also asked for, that he who is fed by God may live in God, and that not only the present and temporal life may be provided for, but the eternal also, to which we may come if our sins are forgiven; and these the Lord calls debts, as He says in His Gospel, "I forgave thee all that debt, because thou desiredst me." And how necessarily, how providently and salutarily, are we admonished that we are sinners, since we are compelled to entreat for our sins, and while pardon is asked for from God, the soul recalls its own consciousness of sin! Lest any one should flatter himself that he is innocent, and by exalting himself should more deeply perish, he is instructed and taught that he sins daily, in that he is bidden to entreat daily for his sins. Thus, moreover, John also in his epistle warns us, and says, "If we say that we have no sin, we deceive ourselves, and the truth is not in us; but if we confess our sins, the Lord is faithful and just to forgive us our sins." In his epistle he has combined both, that we should entreat for our sins, and that we should obtain pardon when we ask. Therefore he said that the Lord was faithful to forgive sins, keeping the faith of His promise; because He who taught us to pray for our debts and sins, has promised that His fatherly mercy and pardon shall follow.
Not only the past, but haply if we have contracted any from this life; because a man, so long as he bears the flesh, cannot but have some at any rate light sins. But these which we call light, do not make light of. If you make light of them when you weigh them, be afraid when you count them. Many light make one huge sin: many drops fill the river; many grains make the lump. And what hope is there? Before all, confession: lest any think himself righteous, and, before the eyes of God who sees that which is, man, that was not and is, lift up the neck. Before all, then, confession; then, love: for of charity what is said? Charity covers a multitude of sins. 1 Peter 4:8 Now let us see whether he commends charity in regard of the sins which subsequently overtake us: because charity alone extinguishes sins. Pride extinguishes charity: therefore humility strengthens charity; charity extinguishes sins. Humility goes along with confession, the humility by which we confess ourselves sinners: this is humility, not to say it with the tongue, as if only to avoid arrogancy, lest we should displease men if we should say that we are righteous. This do the ungodly and insane: I know indeed that I am righteous, but what shall I say before men? If I shall call myself righteous, who will bear it, who tolerate? Let my righteousness be known unto God: I however will say that I am a sinner, but only that I may not be found odious for arrogancy. Tell men what you are, tell God what you are. Because if you tell not God what you are, God condemns what He shall find in you. Would you not that He condemn you? Condemn you. Would you that He forgive? Do you acknowledge, that you may be able to say unto God, Turn Your face from my sins. Say also to Him those words in the same Psalm, For I acknowledge mine iniquity. If we confess our sins, He is faithful and just to forgive us our sins, and to purge us from all iniquity. If we say that we have not sinned, we make Him a liar, and His word is not in us. 1 John 1:9-10 If you shall say, I have not sinned, you make Him a liar, while you wish to make yourself true. How is it possible that God should be a liar, and man true, when the Scripture says the contrary, Every man a liar, God alone true? Romans 3:4 Consequently, God true through Himself, you true through God; because through yourself, a liar.
A hermit said that a man once committed a serious sin. Stricken with remorse, he went to confess to another monk. He did not tell him what he had done, but put it in the form of a question, ‘If such a thought arose in someone’s mind, would he be saved?’ The monk, who had no discretion, answered, ‘You are completely lost.’ When the brother heard this, he said, ‘Well, if I’m going to perish, I’ll go and do it in the world.’ But on his way he considered the matter and decided to tell his temptations to Silvanus, who possessed great discretion in these matters. The brother went to him and did not tell him what he had done, but again put it in the form of a question, ‘If a thought like this came into someone’s mind, could he be saved?’ Silvanus began to speak to him with texts from Scripture, and said, ‘That judgement does not only fall on people tempted to sin.’ The brother perceived the force of the saying, and began to hope, and told him what he had done. When Silvanus learnt what he had done, he acted like a skilled physician and put on his soul a poultice made of texts from Scripture, showing him that repentance is available for all who in truth and in charity turn to God. After some years Silvanus met the monk who had driven the brother to despair, and told him what had happened, and said, ‘That brother, who despaired because of your words, and was going back to the world, is now a bright star among the brothers.’ He told him this so that we may know how perilous it is when anyone confesses thoughts or sins to someone without discretion.
If we acknowledge our sin and confess it, he will forgive it, and not only that one but all our sins. For when a person repents and is baptized into the name of Jesus Christ, God forgives all his sins which he has committed both against himself and against heaven.
If we confess our sins, He is faithful, etc. Because we cannot be without sin in this life, the first hope of salvation is confession, and no one should consider themselves just or lift up their neck before the eyes of God. Then comes love, because charity covers a multitude of sins (1 Peter 4), which is consequently highly recommended to us with much praise in the following parts of this Epistle. Beautifully, both are suggested together, that we should both pray for our sins and obtain God's indulgence when we pray. Therefore, He is also called faithful to forgive sins, retaining the faithfulness of His promise, because He who taught us to pray for our sins and debts promised fatherly mercy and ensuing forgiveness. He also affirms Him to be just, because He justly forgives true confession. "That He may forgive us our sins," he says, "and cleanse us from all iniquity." He forgives daily and slight sins to the elect in this life, without which they cannot live on earth; He cleanses them after the dissolution of the flesh from all iniquity, bringing them into that life in which they neither wish to sin nor can. He now forgives greater temptations to those who pray, so they may not be overcome; He forgives the smallest, so they may not be injured; He then cleanses from all, so that no iniquity at all may exist among the blessed in the eternal kingdom.
To say that God is faithful means that he is reliable, for faithful is a word which is not just applied to those who believe but also to those who can be relied upon. It is in this second sense that it is applied to God. He is also just in that he does not refuse anyone who comes to him, however seriously they may have sinned.
[AD 220] Tertullian on 1 John 1:9