Gregory the Great — on 1John 2:4 (Forty Gospel Homilies, Homily 30)

“Yet in the very beginning of the reading you heard what the Truth says: "If anyone loves me, he will keep my word." The proof of love, therefore, is the showing forth of works. Hence the same John says in his epistle: "He who says, 'I love God,' and does not keep His commandments, is a liar." For we truly love God if we restrain ourselves from our own pleasures for the sake of His commandments. For he who still flows away through illicit desires certainly does not love God, because he contradicts Him by his own will.”