John Calvin Commentary 1 John 1:6

"If we say that we have fellowship with him and walk in the darkness, we lie, and do not the truth:" — 1 John 1:6 (ASV)

If we say—it is, indeed, an argument from what is inconsistent, when he concludes that they are alienated from God, who walk in darkness. This doctrine, however, depends on a higher principle: that God sanctifies all who are his.

For it is not a mere precept that he gives, which requires that our life should be holy. Rather, he shows that the grace of Christ serves this purpose: to dissipate darkness and to kindle in us the light of God. It is as if he had said, “What God communicates to us is not a vain fiction, for it is necessary that the power and effect of this fellowship should shine forth in our life; otherwise, the possession of the gospel is deceptive.”

What he adds, and do not the truth, is the same as if he had said, “We do not act truthfully. We do not regard what is true and right.” This way of speaking, as I have previously observed, is frequently used by him.