Here it should also be noted that in this way God gives Abraham a palpable demonstration of His grace. He had promised him a son, but He delays the fulfillment of the promise. Meanwhile Abraham, who is satisfied with the Word alone, believes the promise and simply clings to the invisible things. But it happens in due time that the invisible things become visible. We, too, should imitate this and set it before our eyes. We believe that our flesh will rise again on the Last Day. This should be as sure for us as if it had already happened; for we, too, have the Word and the same spiritual comforts that Abraham had.
Therefore just as a hundred years ago we were nothing, so when death will have destroyed our flesh, our flesh will come forth again from nothing and will live. Thus the things we have at the present time instruct us clearly about the things we shall have in the future. Let no one say by way of objection that Abraham had the promises; for do we, too, not have the promises in Baptism and in the Eucharist?
The only difference between us and Abraham is this, that Abraham believed the promises, but we do not believe them. Through faith Abraham made visible things out of invisible things, but we cling solely to the visible things and do not feel God palpably in ourselves, as Paul says in Acts 14:17 that He has left in us sure evidences of His presence and goodness.
Nevertheless, Abraham had an advantage that was unique and not to be regarded lightly. We do not have that advantage. He was sure of descendants, because both sons would live. Above, when he was reminded by the Word of God, he learned that twelve princes would spring from Ishmael. He also knew about Isaac that Christ would be born from him. This example we observe in Abraham alone and his family, for no one of us can be sure about the life of his descendants during a single year.23 But this is merely a physical benefit. We, however, so far as spiritual promises are concerned, are either on a par with or even above Abraham; and if we could only believe as Abraham believed, we would be in Paradise, yes, even in heaven itself. For we are baptized, not into the house of Crassus or of Caesar but into the kingdom of heaven. Besides, we have received absolution and the Lord’s Supper. Yet even though we have more promises than Abraham had, we snore in our sluggishness and neither thank God nor rejoice in the spirit.
Hence we do not lack promises, but we lack the faith that Abraham had. Among the papists, however, even doctrine itself has perished; for they teach nothing about faith, nothing about the Word of God, which absolves from sins and gives comfort, and nothing about the promises. They teach only the merits of one’s own works. Baptism is altogether dead among them; they absolve only those who are contrite and have confessed; and they teach those who use the Lord’s Supper that they are doing a work of obedience toward the church. About the remission of sins and about the way of approaching it by faith they teach nothing. But it behooves us to open our eyes, to consider the blessings we have, to give thanks for them, and to rejoice like Abraham, who is glad not only in the spirit but also in the body.
Luther’s works, vol. 4: Lectures on Genesis: Chapters 21-25 p 16-17