Look, the baptized person has received the sacrament of birth. He has a sacrament, and a great sacrament, divine, holy, ineffable. Consider what sort of thing it is. That it makes a new man by remission of all sins. Yet let him focus attention upon the heart, if what has been done in the body has been perfected there. Let him see if he has love and then let him say, "I have been born of God." But if he does not have it, he does indeed have the mark that has been infixed, but he roams at large, a deserter. Let him have love; otherwise let him not say that he has been born of God. "But," he says, "I have the sacrament." Hear the Apostle: "If I should have all sacraments, and have all faith so that I could remove mountains, but have not love, I am nothing."
Augustine, Tractate on First John 5.6, in St. Augustine, Tractates on the Gospel of John 112-24; Tractates on the First Epistle of John, tr. by John W. Rettig, The Catholic University of America Press (Washington, D.C.: 1995), p. 191.