11 “Let that therefore abide in you, which you have heard from the beginning. If that which you have heard from the beginning shall abide in you, you also shall abide in the Son, and in the Father. And this is the promise that He has promised us.” For haply you might ask about the wages, and say, Behold, that which I have heard from the beginning I keep safe in me, I comply therewith; perils, labors, temptations, for the sake of this continuance, I bear up against them all: with what fruit?
What wages? What will He hereafter give me, since in this world I see that I labor among temptations? I see not here that there is any rest: mere mortality weighs down the soul, and the corruptible body presses it down to lower things: but I bear all things, that “that which I have heard from the beginning” may “remain” in me; and that I may say to my God, “Because of the words of Your lips have I kept hard ways.” Unto what wages then? Hear, and faint not. If you were fainting in the labors, upon the promised wages be strong.
Where is the man that shall work in a vineyard, and shall let slip out of his heart the reward he is to receive? Suppose him to have forgotten, his hands fail. The remembrance of the promised wages makes him persevering in the work: and yet he that promised it is a man who can deceive your expectation. How much more strong ought you to be in God's field, when He that promised is the Truth, Who can neither have any successor, nor die, nor deceive him to whom the promise was made!
And what is the promise? Let us see what He has 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.” You have heard, and in your joy you have cried out: love that which you have heard, and you are delivered from your labors into the rest of eternal life.
Lo, this is what God promises; “eternal life.” Lo, this what God threatens; eternal fire. What to those set on the right hand? “Come, you blessed of my Father, receive the kingdom prepared for you from the beginning of the world.” To those on the left, what? “Go into eternal fire, prepared for the devil and his angels.” Thou dost not yet love that: at least fear this.
Source: Homilies on the First Epistle of John (New Advent)