Oecumenius

And we know that the Son of God has come and has given us an understanding, that we may know Him who is true; and we are in Him who is true, in His Son Jesus Christ. This is the true God and eternal life. But since, John says, the Son of God has come and has given us an understanding by which we may know the true God through internal understanding, and be in His true Son Jesus Christ: we have come to know through this glory that He is the true God and eternal life, and by knowing we are safe from the onslaught of evil and its …

Commentary on 1 John ↗

Clement of Alexandria

"And the Son of God has come and given us understanding," which comes to us, that is, by faith, and is also called the Holy Spirit.

From the Latin Translation of Cassiodorus ↗

Augustine of Hippo

And if the Son be not of the same substance as the Father, then is He a made substance: if a made substance, then not "all things were made by Him:" but, "all things were made by Him;" therefore, He is of one and the same substance with the Father. And therefore, not only God, but True (or, Very) God. Which the same John doth most openly affirm in his epistle: "We know that the Son of God is come; and hath given us an understanding that we may learn to know the True God, and may be in His true Son Jesus Christ. …

Ten Homilies on 1 John 10 ↗

Bede

And we know that the Son of God has come, etc. For what could be clearer than these words? What could be sweeter? What could be said more strongly against all heresies? Christ is the true Son of God. The Father of Christ Jesus our Lord is the true God. The eternal Son of God came temporally into the world, who was in the world, and through whom the world was made. Nor did He come for any other reason than for our salvation, that is, to give us the understanding to know the true God. …

Commentary on the Catholic Epistles ↗