Cyril of Jerusalem

We can turn this around and say that anyone who despises the Begotten also despises the One who begat him.

Catechetical Lecture 11:7 ↗

Theophylact of Ohrid

And this is said in confirmation of the above. If we received from the Teacher the commandment to love one another, then, without a doubt, if we believe that our Teacher Jesus is the Christ, and He is the Christ in the sense that He is God and man together, we must also keep His commandments, as those of a Teacher and as those of God. …

Commentary on 1 John ↗

Oecumenius

Everyone who believes that Jesus is the Christ is born of God. And whoever loves the one who created him loves also the one who is born of him. John confirms the discourse again with others, thereby strengthening it in a certain way: If we have received the command from the Master to love one another mutually, surely if we believe that our Master and Teacher is Jesus Christ, who is Christ in the same way that God and man are: we shall also observe his commandments as both a Teacher and as God. …

Commentary on 1 John ↗

Augustine of Hippo

But, that we might run by Him, He reached even unto us: for we were afar off, foreigners in a far country. Not enough that we were in a far country, we were feeble also that we could not stir. A Physician, He came to the sick: a Way, He extended Himself to them that were in a far country. Let us be saved by Him, let us walk in Him. This it is to "believe that Jesus is the Christ," as Christians believe, who are not Christians only in name, but in deeds and in life, not as the devils believe. …

Ten Homilies on 1 John 10 ↗

Bede

And everyone who loves the one who begot, loves also the one, etc. With marvelous skill in preaching, blessed John took care to incite us to the love of our neighbor, first noting that everyone who perfectly believes is born of God, then suggesting how just it is that the one who loves God should love also the one born of God. …

Commentary on the Catholic Epistles ↗