1 John 1:6

...the apostles who did also write the Gospel, in which they recorded the doctrine regarding God, pointing out that our Lord Jesus Christ is the truth, and that no lie is in Him. As also David says, prophesying His birth from a virgin, and the resurrection from the dead, "Truth has sprung out of the earth." The apostles, likewise, being disciples of the truth, are above all falsehood; for a lie has no fellowship with the truth, just as darkness has none with light, but the presence of the one shuts out that of the other. Our Lord, therefore, being the truth, did not speak lies...

And how can they say that they alone know God when they do the same things as those who are loathed by the heathen because they do not do what the laws direct, that is, as the wicked and incontinent and covetous and adulterous? They ought to live good lives even while they are dwelling in an alien country, to manifest their truly kingly nature. 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." How then are they who do these things superior to worldly men when they behave like the very worst men of this world? Those whose actions are alike are in my opinion of like nature.

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.

The truth is light, so if we do not walk according to it, we are in the dark.

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.

John says that if we sin we have no fellowship with God, but if we do the works of light, then we have fellowship with one another.

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).

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.

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.