An Excerpt from Martin Luther's Babylonian Captivity of the Church:
For just as the truth of this divine promise, once pronounced over us, continues until death, so our faith in it ought never to cease, but to be nourished and strengthened until death by the continual remembrance of this promise made to us in baptism. Therefore, when we rise from our sins or repent, we are merely returning to the power and the faith of baptism from which we fell, and finding our way back to the promise then made to us, which we deserted when we sinned. For the truth of the promise once made remains steadfast, always ready to receive us back with open arms when we return. And this, if I mistake not, is what they mean when they say, though obscurely, that baptism is the first sacrament and the foundation of all the others, without which none of the others can be received.
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 (II Tim. 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 [Heb. 10:23; 11:11], and I received his sign in baptism. If God is for me, who is against me?” [Rom. 8:31].
Martin Luther, Luther’s Works, Vol. 36: Word and Sacrament II, ed. Jaroslav Jan Pelikan, Hilton C. Oswald, and Helmut T. Lehmann, vol. 36 (Philadelphia: Fortress Press, 1999), 59–60.