Clement of Alexandria

"If we say that we have not sinned, we make Him a liar, and His word is not in us." His doctrine, that is, or word is truth.

From the Latin Translation of Cassiodorus ↗

Augustine of Hippo

"If we say that we have not sinned, we make Him a liar, and His word is not in us." If thou shalt say, I have not sinned, thou makest Him a liar, while thou wishest to make thyself true. How is it possible that God should be a liar, and man true, when the Scripture saith the contrary, "Every man a liar, God alone true"? Consequently, God true through Himself, thou true through God; because through thyself, a liar.

Ten Homilies on 1 John 1 ↗

Bede

If we say that we have not sinned, etc. For he himself said through a man filled with his Spirit: There is no just man on earth who does good and does not sin (Ecclesiastes 7). But even through himself, he taught us that we cannot be free from sins, as he commanded us to pray thus: Forgive us our debts as we forgive our debtors (Matthew 6). Therefore, no one, being taught by Pelagius, should believe that he can live free from sins and debts, when he sees the apostles, being taught by the Lord, praying for their own sins. …

Commentary on the Catholic Epistles ↗