10 “If any man say, I love God.” What God? wherefore love we? “Because He first loved us,” and gave us to love. He loved us ungodly, to make us godly; loved us unrighteous, to make us righteous; loved us sick, to make us whole. Ask each several man; let him tell you if he love God. He cries out, he confesses: I love, God knows. There is another question to be asked. “If any man say, I love God, and hates his brother, he is a liar.” By what do you prove that he is a liar?
Hear. “For he that loves not his brother whom he sees, how can he love God whom he sees not?” What then? Does he that loves a brother, love God also? He must of necessity love God, must of necessity love Him that is Love itself. Can one love his brother, and not love Love? Of necessity he must love Love. What then? Because he loves Love, does it follow that he loves God? Certainly it does follow. In loving Love, he loves God. Or have you forgotten what you said a little while ago, “Love is God”? If “Love is God,” whoso loves Love, loves God.
Love then your brother, and feel yourself assured. You can not say, “I love my brother, but I do not love God.” As you lie, if you say, “I love God,” when you love not your brother, so you are deceived when you say, I love my brother, if you think that you love not God. Of necessity must you who lovest your brother, love Love itself: but “Love is God:” therefore of necessity must he love God, whoso loves his brother. But if you love not the brother whom you see, how can you love God whom you see not?
Why does he not see God? Because he has not Love itself. That he does not see God, is, because he has not love: that he has not love, is, because he loves not his brother. The reason then why he does not see God, is, that he has not Love. For if he have Love, he sees God, for “Love is God:” and that eye is becoming more and more purged by love, to see that Unchangeable Substance, in the presence of which he shall always rejoice, which he shall enjoy to everlasting, when he is joined with the angels.
Only, let him run now, that he may at last have gladness in his own country. Let him not love his pilgrimage, not love the way: let all be bitter save Him that calls us, until we hold Him fast, and say what is said in the Psalm: “You have destroyed all that go a-whoring from You” — and who are they that go a-whoring? They that go away and love the world: but what shall you do? He goes on and says:— “but for me it is good to cleave to God.” All my good is, to cling unto God, freely. For if you question him and say, For what do you cling to Him? And he should say, That He may give me— Give you what? It is He that made the heaven, He that made the earth: what shall He give you? Already you are cleaving to Him: find something better, and He shall give it you.
Source: Homilies on the First Epistle of John (New Advent)