Oecumenius

"And this is the promise that he himself made to us, eternal life." "And this is the promise." For the conjunction και, that is, "and", must be taken causally for "indeed". But what is the promise? It is that which the Lord says to the Father: "That as I am in you and you are in me, so they may be one in us." (Jn. 17:21) And again: "That they may have eternal life. And this is eternal life, that they may know you, the only true God, and Jesus Christ whom you have sent." (Jn. 17:3)

Commentary on 1 John ↗

Augustine of Hippo

And what is the promise? Let us see what He hath promised. Is it gold which men here love much, or silver? Or possessions, for which men lavish gold, however much they love gold? Or pleasant lands, spacious houses, many slaves, numerous beasts? Not these are the wages, so to say, for which he exhorts us to endure in labor. What are these wages called? "eternal life." Ye have heard, and in your joy ye have cried out: love that which ye have heard, and ye are delivered from your labors into the rest of eternal life. …

Ten Homilies on 1 John 3 ↗

Bede

And this is the promise which He Himself promised us, eternal life. As if you were asking for a reward, and you were saying: "Behold me, I keep what I have heard from the beginning, I obey; I endure dangers, labors, temptations, for maintaining this. What will be the fruit, what the reward? What will be given to me afterward?" And this, he says, is the promise which He Himself promised us, eternal life. Let the remembrance of the promised reward make you perseverant in your work.

Commentary on the Catholic Epistles ↗