Theophylact of Ohrid

Having said that the Son of God appeared and that He appeared in order to, having committed no sin of His own, take away, that is, destroy our sin, he adds: "everyone who sins has not seen Him." If you, he says, when He appeared, saw Him, and as a result of seeing Him received the power not to yield to sin, as being perfectly established in Him, then it is obvious that those who sin have not seen Him, and therefore have not known Him. …

Commentary on 1 John ↗

Augustine of Hippo

"Whosoever abideth in Him, sinneth not." In so far as he abideth in Him, in so far sinneth not. "Whosoever sinneth hath not seen Him, neither known Him." A great question this: "Whosoever sinneth hath not seen Him, neither known Him." No marvel. We have not seen Him, but are to see; have not known Him, but are to know: we believe on One we have not known. Or haply, by faith we have known, and by actual beholding have not yet known? But then in faith we have both seen and known. …

Ten Homilies on 1 John 4 ↗

Bede

And everyone who sins has neither seen Him nor known Him. He speaks of the vision and knowledge of faith, with which the righteous even in this life delight in seeing God, until they reach the very form of His open vision in the future, of which it is said above: For we will see Him as He is. Therefore, everyone who sins has neither seen Him nor known Him. For if he had tasted and seen how sweet the Lord is (Psalm 34), he would by no means separate himself from the vision of His glory by sinning. …

Commentary on the Catholic Epistles ↗