Oecumenius

And you know that He appeared to take away our sins: and in Him, there is no sin. Everyone who abides in Him does not sin. Everyone who sins has not seen Him nor known Him. "And you know that He appeared." You can no longer have a place for sinning. For when Christ came to take away our sins, since he was not a partaker of sin, it has not been left to you to sin any longer, you who have come to him and have been confirmed in his faith. …

Commentary on 1 John ↗

Augustine of Hippo

Well then, what are we to do concerning sins and iniquities? Hear what He saith: "And ye know that He was manifested to take away sin; and sin in Him is not." He, in Whom sin is not, the same is come to take away sin. For were there sin in Him, it must be taken away from Him, not He take it away Himself.

Ten Homilies on 1 John 4 ↗

Bede

And you know that He appeared to take away sins, etc. John the Baptist also bore witness to this about the Lord, saying: Behold the Lamb of God, who takes away the sins of the world. The Lord was able to take away sins because there was no sin in Him. Many and so great men came into the world perfect, but none of them could take away the sins of the world, because none could be without sin in the world. …

Commentary on the Catholic Epistles ↗