The Greek is very emphatic here — not a fleck, not a spot. Here John strikes the great error which seemed to ascribe both good and evil to God. If good and evil were bound up in God, if sin and holiness were bound up in God, then man could consistently say. "I have participation and fellowship with God, even if I live in conscious, daily sin, for God is a being of mixed character, and I am like Him in that sense." Now, in inculcating holiness upon men, John must see that the conception of the model is right, and hence he aims to clear the character of God of all such false conceptions; and this is the way he starts out, this is the message — "that God is light," undimmed, unmixed light, in whom is no darkness at all, not a fleck of darkness nor of evil nor of sin in His nature. You see John does not prove this, he simply asserts it; that is the style of John, the nearest writer to the Lord Jesus in his form of expression. The Lord Jesus did not often reason, but spoke from authority, gave expression to His own intuitions of truth. And John, as an intuitive writer, simply announces his intuitions, St. Paul reasons — has long and involved chains of argument. Hence John makes the declaration, under the illumination of the divine Spirit, that God is unmixed in His character, a being of unmixed holiness, love, truth, and in Him is nothing to the contrary. Now, then, he can clinch his nail. "If we say that we have fellowship with Him, and walk in darkness," in an element that is directly contrary to His character, there is a great mistake somewhere, a falsehood somewhere; the truth is not in the utterance. "If we say that we have fellowship with Him," participation of His moral likeness, "and" still "walk in darkness," walk in sin, in untruth, "we lie," John is a very outspoken writer; he does not mince matters and say we are mistaken and do not the truth, do not exemplify the truth, do not live out the truth, but, "we lie, and do not the truth." He then goes on: "But if we walk in the light," abstain from sin, are victorious over sin, "if we walk in the light as He is in the light, we have fellowship one with another, and the blood of Jesus Christ His Son cleanseth us from all sin."
— Mile-Stone Papers, Part 1, Chapter 11.