4 “We are of God.” Let us see why; see whether it be for any other thing than charity. “We are of God: he that knows God hears us; he that is not of God hears not us. Hereby know we the spirit of truth, and of error:” namely by this, that he that hears us has the spirit of truth; he that hears not us, has the spirit of error. Let us see what he advises, and let us choose rather to hear him advising in the spirit of truth, and not antichrists, not lovers of the world, not the world.
If we are born of God, “beloved,” he goes on— see above from what: “We are of God: he that knows God hears us; he that is not of God hears not us. Hereby know we the spirit of truth, and of error:” aye, now, he makes us eagerly attentive: to be told that he who knows God, hears; but he who knows not, hears not; and that this is the discerning between the spirit of truth and the spirit of error: well then, let us see what he is about to advise; in what we must hear him— “Beloved, let us love one another.” Why?
Because a man advises? “Because love is of God.” Much has he commended love, in that he has said, “Is of God:” but he is going to say more; let us eagerly hear. At present he has said, “Love is of God; and every one that loves is born of God, and knows God. He that loves not knows not God.” Why? “For God is love” [Love is God]. What more could be said, brethren? If nothing were said in praise of love throughout the pages of this epistle, if nothing whatever throughout the other pages of the Scriptures, and this one only thing were all we were told by the voice of the Spirit of God, “For Love is God;” nothing more ought we to require.
Source: Homilies on the First Epistle of John (New Advent)