It will therefore be no small gain to a penitent to remember above all his baptism, and, confidently calling to mind the divine promise which he has forsaken, acknowledge that promise before his Lord, rejoicing that he is still within the fortress of salvation because he has been baptized, and abhorring his wicked ingratitude in falling away from its faith and truth. His heart will find wonderful comfort and will be encouraged to hope for mercy when he considers that the promise which God made to him, which cannot possibly lie, is still unbroken and unchanged, and indeed, cannot be changed by sins, as Paul says (2 Timothy 2:13): “If we are faithless, he remains faithful—for he cannot deny himself.” This truth of God, I say, will sustain him, so that if all else should fail, this truth, if he believes in it, will not fail him. In it the penitent has a shield against all assaults of the scornful enemy, an answer to the sins that disturb his conscience, an antidote for the dread of death and judgment, and a comfort in every temptation—namely, this one truth—when he says: “God is faithful in his promises [Hebrews 10:23; 11:11], and I received his sign in baptism. If God is for me, who is against me?” [Romans 8:31].
– Martin Luther, “The Babylonian Captivity of the Church”, LW 36:59