But because they have chosen to disobey the laws, they make themselves objects of hatred both to human lawgivers and to the law of God. At any rate in Numbers the man who thrust his spear into the fornicator is evidently blessed by God. And John says in his epistle: "If we say that we have fellowship with him and walk in darkness, we lie and do not the truth; but if we walk in the light as he is in the light, we have fellowship with him, and the blood of Jesus his Son cleanses us from sin."
If "God be light, and in Him is no darkness at all, and we must have fellowship with Him," then from us also must the darkness be driven away, that there may be light created in us, for darkness cannot have fellowship with light. "If we say that we have fellowship with Him, and walk in darkness, we lie." Thou sayest thou hast fellowship with God, and thou walkest in darkness; "and God is light, and in Him is no darkness at all:" then how should there be fellowship between light and darkness? At this point therefore a man may say to himself, What shall I do? how shall I be light? I live in sins and iniquities. There steals upon him, as it were, a desperation and sadness. There is no salvation save in the fellowship of God. "God is light, and in Him is no darkness at all." But sins are darkness, as the Apostle saith of the devil and his angels, that they are "rulers of this darkness." He would not call them of darkness, save as rulers of sins, having lordship over the wicked.
If we say that we have fellowship with Him, etc. He names darkness, sins, heresies, and hatreds. Therefore, the mere confession of faith alone is not sufficient for salvation, to which the testimony of good works is lacking. But neither does the rectitude of works without the simplicity of faith and love profit. For whoever is besieged by darkness in any part cannot have fellowship with Him in whom no iniquities occur. For what fellowship has light with darkness? (II Cor. VI).
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.
See to it, brothers, that while we seem to be in God and think that we have communion with him, we should not be found excluded and separated from him, since we do not now see his light.
[AD 202] Irenaeus on 1 John 1:6