Genesis 26:4

Observe, too, how the same promises are made to Isaac and to Jacob. For thus He speaks to Isaac: "And in thy seed shall all the nations of the earth be blessed." And to Jacob: "And in thee and in thy seed shall all families of the earth be blessed." He says that neither to Esau nor to Reuben, nor to any other; only to those of whom the Christ should arise, according to the dispensation, through the Virgin Mary. But if you would consider the blessing of Judah, you would perceive what I say. For the seed is divided from Jacob, and comes down through Judah, and Phares, and Jesse, and David. And this was a symbol of the fact that some of your nation would be found children of Abraham, and found, too, in the lot of Christ; but that others, who are indeed children of Abraham, would be like the sand on the sea-shore, barren and fruitless, much in quantity, and without number indeed, but bearing no fruit whatever, and only drinking the water of the sea.

Then he teaches Isaac what it was he promised and the things about which God had given him confirmation. "I will make your descendants as numerous as the stars of heaven." This, you remember, he had said to the patriarch as well, that his descendants would be so numerous as to be compared with the stars and the sand: "I will give all this land to your descendants," he said, "and in your descendants will all the nations of the earth be blessed." Hence the promises made to him I will fulfill in you, "for the reason that your father Abraham obeyed my voice and kept my commands, my judgments and my laws." See God's wisdom in stirring up the good man's thinking, making him more enthusiastic and having the effect of rendering him an imitator of his father. After all, God is saying, if Abraham was judged worthy of such a wonderful promise for obeying my voice, on account of his virtue I am about to fulfill it in you, the child born to him. And if you yourself become an imitator of him and tread in his path, consider the degree of favor you will enjoy from me and the care you will be accorded. I mean, the man destined to prosper for someone else's virtue will be accorded much greater favor if he himself proves virtuous.

For this is that nation which God of old promised to Abraham, when He declared that He would make him a father of many nations; not meaning, however, the Arabians, or Egyptians, or Idumaeans, since Ishmael became the father of a mighty nation, and so did Esau; and there is now a great multitude of Ammonites. Noah, moreover, was the father of Abraham, and in fact of all men; and others were the progenitors of others. What larger measure of grace, then, did Christ bestow on Abraham? This, namely, that He called him with His voice by the like calling, telling him to quit the land wherein he dwelt. And He has called all of us by that voice, and we have left already the way of living in which we used to spend our days, passing our time in evil after the fashions of the other inhabitants of the earth; and along with Abraham we shall inherit the holy land, when we shall receive the inheritance for an endless eternity, being children of Abraham through the like faith. For as he believed the voice of God, and it was imputed to him for righteousness, in like manner we having believed God's voice spoken by the apostles of Christ, and promulgated to us by the prophets, have renounced even to death all the things of the world. Accordingly, He promises to him a nation of similar faith, God-fearing, righteous, and delighting the Father; but it is not you, "in whom is no faith."