Gregory the Great — on 1John 2:4 (Forty Gospel Homilies, Homily 29)

“For true faith is that which does not contradict in conduct what it says in words. Hence it is that Paul says of certain false believers: "They profess to know God, but they deny Him by their deeds." Hence John says: "He who says he knows God and does not keep His commandments is a liar." Since this is so, we ought to recognize the truth of our faith in the consideration of our life. For then we are truly faithful, if we fulfill in works what we promise in words. On the day of baptism indeed, we promised to renounce all the works of the ancient enemy and all his pomps. Therefore let each of you bring back the eyes of his mind to consideration of himself; and if he keeps after baptism what he pledged before baptism, let him rejoice, now certain that he is faithful. But behold, if he has by no means kept what he promised, if he has slipped into practicing wicked works, into desiring the pomps of the world; let us see if he now knows how to lament that he has erred. For before the merciful Judge, not even he will be held a deceiver who returns to the truth, even after he has lied, because almighty God, when He willingly receives our repentance, Himself hides by His own judgment that in which we have erred.”