From City of God, 16.31 (ca. 426)
Yet Abraham is worthy of praise, because he all along believed that his son, on being offered up, would rise again. For God had said to him, when he was unwilling to fulfill his wife’s pleasure by casting out the bond maid and her son, “In Isaac shall your seed be called.” No doubt, God then goes on to say, “And as for the son of this bond woman, I will make him a great nation, because he is your seed.” How then is it said “In Isaac shall your seed be called,” when God calls Ishmael also his seed?
Paul, in explaining this, says, “In Isaac shall your seed be called, that is, those who are the children of the flesh, these are not the children of God: but the children of the promise are counted for the seed.” In order, then, that the children of the promise may be the seed of Abraham, they are called in Isaac, that is, they are gathered together in Christ by the call of grace.
Therefore the father, holding fast from the first the promise which fittingly came to be fulfilled through this son whom God had ordered him to sacrifice, did not doubt that this son whom he once thought it hopeless he should ever receive again would be restored to him when he had offered him up. It is in this way the passage in the Epistle to the Hebrews is also to be understood and explained. “By faith,” he says, “Abraham overcame, when tempted about Isaac: and he who had received the promise offered up his only son, to whom it was said, in Isaac shall your seed be called, thinking that God was able to raise him up, even from the dead.”
Therefore Paul has added, “from whence also he received him in likeness.” In whose likeness but his of whom the apostle Paul says, “He who spared not his own Son, but delivered him up for us all?” And on this account Isaac also himself carried to the place of sacrifice the wood on which he was to be offered up, just as the Lord himself carried his own cross. Finally, since Isaac was not to be slain, after his father was forbidden to smite him, who was that ram, the one which Abraham offered by which the sacrifice was completed with foreshadowing blood? For when Abraham saw him, the ram was caught by the horns in a thicket. What, then, did he represent but Jesus, who, before he was offered up, was crowned with thorns by the Jews?
St. Augustine (354-430) was a theologian and philosopher who served as Bishop of Hippo Regius in North Africa. He was a voluminous author, whose writings about God’s grace, the Sacraments, and the Church have been profoundly influential in the development of Western Christianity. The City of God is his master work, a defense of Christian teaching that deeply probes topics like human nature, sin and grace, and God’s purposes in history. His feast day is August 26.