If we confess our sins, faithful and just is He to remit them to us, and utterly purify us from every unrighteousness." Does he say "from impurity? "(No): or else, if that is so, then (He "utterly purifies" us) from "idolatry" too.
Let us then acknowledge, beloved brethren, the wholesome gift of the divine mercy; and let us, who cannot be without some wound of conscience, heal our wounds by the spiritual remedies for the cleansing and purging of our sins. Nor let any one so flatter himself with the notion of a pure and immaculate heart, as, in dependence on his own innocence, to think that the medicine needs not to be applied to his wounds; since it is written, "Who shall boast that he hath a clean heart, or who shall boast that he is pure from sins? " And again, in his epistle, John lays it down, and says, "If we say that we have no sin, we deceive ourselves, and the truth is not in us." But if no one can be without sin, and whoever should say that he is without fault is either proud or foolish, how needful, how kind is the divine mercy, which, knowing that there are still found some wounds in those that have been healed, even after their healing, has given wholesome remedies for the curing and healing of their wounds anew!
That no one is without filth and without sin. In Job: "For who is pure from filth? Not one; even if his life be of one day on the earth." Also in the fiftieth Psalm: "Behold, I was conceived in iniquities, and in sins hath my mother conceived me." Also in the Epistle of John: "If we say that we have no sin, we deceive ourselves, and the truth is not in us."
Since God is light, there is no darkness in him at all, and he has nothing to do with darkness. The person who is enlightened by his light walks in the light, according to the words of the Savior himself: “While you have the light, walk in the light, lest the darkness take hold of you.” Anyone who walks in the darkness of sin but claims that his mind is not darkened and that he has a relationship with God is lying.
"If we say that we have no sin, we deceive ourselves, and the truth is not in us." Consequently, if thou hast confessed thyself a sinner, the truth is in thee: for the Truth itself is light. Thy life hath not yet shone in perfect brightness, because there are sins in thee; but yet thou hast already begun to be enlightened, because there is in thee the confession of sins.
Wisdom availeth more in the way of help than a band of the most powerful men in a city, and it often also pardons righteously those who fail in duty. For there is not one that stumbleth not.
This is how John describes the Jews who dared to say: “Let his blood be upon us and upon our children,” as if they could crucify Christ without being sinners. If we say such things, we deceive ourselves.
If we say that we have no sin, etc. This sentence avails against the heresy of Pelagius, which claimed that all infants are born without sin and that the elect in this life can progress to such an extent as to be without sin. For even when the prophet says: "Behold, I was conceived in iniquity, and in sin my mother bore me" (Psalm 50), we cannot be without guilt in the world, having come into the world with guilt. But the blood of Jesus Christ, the Son of God, cleanses us from all sin so that our debts do not hold us under the power of our enemy, because the Mediator between God and man, the man Christ Jesus, freely repaid on our behalf what He did not owe. For He who undeservedly paid the debt of death in the flesh for us, freed us from the debt of the death of the soul.
If we say that we have no sin, we deceive ourselves, and the truth is not in us. If we confess our sins, he is faithful and just to forgive us of our sins and to cleanse us from all unrighteousness. If we say that we have not sinned, we make him a liar, and his word is not in us.
"If we confess our sins." John repeats the statement, using it more richly, so that the frequency of the produced refutation may restrain them by the degree of sin itself and invite them to confession. However, John indicated how much good arises from confession when he said: "Say your sins first, so that you may be justified." (Is. 43:26) However, it was also the custom of the Teacher of this beloved disciple to repeat the same things more frequently, first moderately, then more perfectly, wishing to make the understanding of the words more effective, and striving to instill their habit into the listeners.
Indeed, John called God faithful. This means “truthful”. For "Πιστός", that is, “faithful”, is said not only of one to whom something is entrusted, but also of one who is confirmed in truth, who by his truthful manner causes others to become partakers of this as well. Therefore, God is called faithful in this way: just, indeed, as one who does not reject those who approach him, regardless of the sins they have committed. Therefore, he undoubtedly forgives the sins of those who run to holy baptism through repentance, whether they have sinned against him or against another.
There, if we confess, John says, we shall obtain appropriate forgiveness. But if we shamelessly say that we have not sinned, we shall commit a double evil, both showing ourselves to be liars and slandering God, who surpasses all truth, with falsehood. For He Himself says through the Prophet: “They have repaid me evil for good.” (Ps. 109:5)
And again from John’s own mouth: “If I have spoken wrongly, testify to the wrong; but if rightly, why do you strike me?” (Jn. 18:23) If, with these things being so, we still say that we have not sinned, we deny his words, which are spirit and life. (Jn. 6:64) "For the words that I speak," he says, "are spirit and life." And if we no longer have his word dwelling in us: what could be more serious?
[AD 220] Tertullian on 1 John 1:8