1 John 1:7

"And the blood of Jesus Christ His Son," he says, "cleanses us." For the doctrine of the Lord, which is very powerful, is called His blood.

From the Epistle also of John they forthwith cull (a proof). It is said: "The blood of His Son purifieth us utterly from every sin." Always then, and in every form, we will sin, if always and from every sin He utterly purifies us; or else, if not always, not again after believing; and if not from sin, not again from fornication.

"If we walk in the light, as He is in the light, we have fellowship one with another." Let us walk in the light, as He is in the light, that we may be able to have fellowship with Him. And what are we to do about our sins? Hear what follows, "And the blood of Jesus Christ His Son shall purge us from all sin." Great assurance hath God given! Well may we celebrate the Passover, wherein was shed the blood of the Lord, by which we are cleansed "from all sin!" Let us be assured: the "handwriting which was against us," the bond of our slavery, the devil held, but by the blood of Christ it is blotted out. "The blood," saith he, "of His Son shall purge us from all sin." What meaneth, "from all sin"? Mark: even now, in the name of Christ whom these here have now confessed, who are called infants, have all their sins been cleansed. They came in old, they went out new. How, came in old, went out new? Old men they came in, infants they went out. For the old life is old age with all its dotage, but the new life is the infancy of regeneration.

The blood of animal sacrifices was enough to cleanse people from whatever particular sin they had committed, but the blood of Christ is sufficient to cleanse those who walk in love from all sin.

If we say that we have fellowship with Him and walk in darkness, we lie and do not practice the truth. But if we walk in the light, as He is in the light, we have fellowship with one another, and the blood of Jesus Christ His Son cleanses us from all sin.
Since therefore we are called to have fellowship with God, who is light; furthermore, in such light the darkness cannot stand, as has been demonstrated, nor can we, who are partakers of the light, thereafter receive darkness in ourselves, lest we suffer the penalties of falsehood and be separated from the fellowship of light along with falsehood. Therefore, having mutual fellowship (namely, both among ourselves and with the light), we make ourselves difficult to sin.
But how will this happen to us, John says, who have previously offended in many sins? For no one who is truthful and accustomed to speaking the truth will dare to say that he is without sin. Therefore, if anyone is surrounded and occupied by this fear, let him be of good spirit, he says, for he has been purified by the blood of His Son Jesus Christ shed for us, having contracted a fellowship with Him. Therefore, observe that because of the supreme union, He also calls the Son of the Father the nature assumed from us, whose blood is undoubtedly of the assumed nature and not of God.
How then, is Nestorius mad and impious, who separating the flesh of the Son does not allow His mother to be called Θεοτόκον, that is, the Mother of God? "But if we walk in the light, as He is in the light," (1 Jn. 1:7) it should be understood that the understanding of the whole sentence is the overturning of the blasphemy of the Jews (see Rev. 2:9): for they indeed said of Christ: "We know that He is a sinner." (Jn. 9:34) John says therefore: If we do the works of light, we are partakers of him; but if we do not, we are strangers to him. How then, is he not truthful, he asks, and has committed no sin, although "he was rejected by us among sinners?" (Is. 53:12) If therefore we who said, "His blood be upon us and upon our children," (Matt. 27:24) boldly say that we have not sinned, we deceive ourselves (1 Jn. 1:8) or are deceived, as if it were not a sin to crucify Christ.

But if we walk in the light, etc. The distinction of words is to be noted, because he says that God is in the light, but that we ought to walk in the light. For the righteous walk in the light, as they advance to better things by serving with works of virtue. But divine holiness, to which it is said: "But you are the same" (Ps. CI), is rightly remembered to be in the light, because being full goodness always existing, it does not find where it may advance. To the faithful it is said: "Walk as children of light" (Eph. V). For the fruit of light is in all goodness and righteousness and truth. But God always exists as good, just, and true without any progress. If, therefore, we walk in the light, as He is in the light, we have fellowship with one another. Because it evidently gives an indication that we are advancing by walking in the path of light, if we rejoice in the bond of fraternal fellowship, with which we likewise may reach the true light. But even if we are found to do the works of light, if we seem to hold inviolable the rights of mutual love, we should not think that by our progress or industry we are able to be completely cleansed from sins. For it follows:

And the blood of Jesus Christ, His Son, cleanses us from all sin. For indeed, the sacrament of the Lord's passion both alleviated all past sins for us in baptism and forgives whatever we commit after baptism due to daily frailty, by the grace of the same our Redeemer. Especially when among the works of light that we perform, we humbly confess our errors to Him daily, when we receive the sacraments of His blood, when we pray for our debts to be forgiven as we forgive our debtors, when, mindful of His passion, we willingly endure each adverse thing. Remarkably, when speaking of the Lord, it is said: "And the blood of Jesus His Son"; indeed, the Son of God in the nature of divinity could not have blood; but since the same Son of God also became the Son of man, appropriately He calls the blood of the Son of God due to the unity of His person, to show that He truly assumed a body, truly shed His blood for us; and to refute the heretics who either deny that the true flesh was assumed by the Son of God or that the Lord Jesus truly suffered in the flesh which He assumed. Similar to this is what Paul says: "The Holy Spirit has made you overseers to shepherd the church of God which He purchased with His own blood" (Acts 20).

The Apostle again returns to his former discourse and explains what Gospel he heard, namely the following: God is light, and there is no darkness in Him. From whom did he hear this? From Christ Himself, Who said: "I am the light of the world" (John 8:12), and again: "I have come as a light into the world" (John 12:46). So then, He is light, and there is no darkness in Him, but a spiritual light that draws the eyes of the soul to behold Him, turning them away from everything material and arousing desire for Him alone with the most fervent love. By "darkness" he means either ignorance or sin; for in God there is neither ignorance nor sin, because ignorance and sin have their place in matter and in our disposition. And if it is said somewhere: "He made darkness His covering" (Ps. 18:12), it says that He "made" darkness, not that He "is" darkness, as it says He "is" light. For that which sets in place and that which is set in place are not the same thing. So here "darkness" signifies our ignorance about God, on account of His incomprehensibility, and this ignorance is ours, not God's. For sometimes something is attributed to one in whom it does not exist, not for his own sake, but for the sake of someone who has a relation to him. And that the Apostle calls sin darkness is evident from his Gospel saying: "and the light shines in the darkness, and the darkness did not overcome it" (John 1:5), where by darkness he means our sinful nature, which by its inclination toward falling yields to our envious adversary the devil, who drags us into sin. So then, the Light, having united Himself with our nature, which is so easily seized, became completely unseizable by the tempter. For "He committed no sin" (Isa. 53:9). So then, when we receive you as sharers in communion with God, Who is light, and in this light, as has been shown, there can be no darkness, then we too, as sharers of the light, must not admit darkness into ourselves, lest we suffer punishment for falsehood and together with falsehood be cut off from communion with the light. Therefore, holding to communion with one another, that is, with us and with the light, we must make ourselves unconquerable by sin. But how will this be, when we have already been mired in many sins before? For no one who loves truth and strives to be truthful will dare to say that he is without sin. So then, if anyone is seized by this fear, let him not despair: for whoever has entered into communion with His Son Jesus Christ has been cleansed by His Blood, shed for us. Note that on account of the most intimate union he calls Him the Son of the Father even according to what was assumed by Him from us; for blood, without doubt, belongs to our nature, not to God. And Nestorius is clearly insane and impious when he separates the flesh from the Son and does not allow His Mother to be called the Theotokos. One must also know that the entire thought of this passage overthrows the blasphemy of the Jews as well, who said: "We know that this Man is a sinner" (John 9:24). So then, he says, if we do the works of light, then we are in communion with Him, but if we do not do them, then we are strangers to Him. And how is He not the true light and completely sinless, when He "was numbered with the transgressors" for your sake (Isa. 53:12)? So then, if we, who once cried out: "His blood be on us and on our children" (Matt. 27:25), shamelessly say that we have not sinned, then we "deceive ourselves," as though crucifying Christ were no sin. The Apostle did not say: we lie, but: we deceive ourselves, because deception is outside of truth. But if we acknowledge our sin and confess it, He will forgive us.

We have a strange illusion that mere time cancels sin. I have heard others, and I have heard myself, recounting cruelties and falsehoods committed in boyhood as if they were no concern of the present speaker's, and even with laughter. But mere time does nothing either to the fact or to the guilt of a sin. The guilt is washed out not by time but by repentance and the blood of Christ: if we have repented these early sins we should remember the price of our forgiveness and be humble.