Genesis 13:16

Truly that multitude which was promised to Abraham is not innumerable to God, although it is to the human mind. But to God not even the dust of the earth is so. Further, the promise here made may be understood not only of the nation of Israel but of the whole seed of Abraham, which may be fitly compared with the dust for its multitude. Regarding this seed, there is also the promise of many children, not according to the flesh but according to the spirit. However, the reason why I said that this is not clear from the text is that even the multitude of the one people sprung from Abraham, according to the flesh, through his grandson Jacob, has increased so greatly as to fill almost every region of the world. It is because even the number of this progeny is beyond human power to count that it may, by a hyperbole, be compared with the number of dust particles. What is beyond doubt is that the only land meant is that which is called Canaan. However, some may find a difficulty in the expression "I will give to you and your posterity forever," if the "forever" is taken to mean "eternally." There is no trouble if only they will take this "forever" to mean "to the end of time," which, as we hold on faith, is to be the beginning of eternity. For although the Israelites are expelled from Jerusalem, they still remain in other cities in the land of Canaan and shall remain even to the end. And even when that whole land is inhabited by Christians, they also are the very seed of Abraham.

Then, in case Abraham should have regard only to his own condition, his advanced years and Sarah's sterility, and thus lose confidence in the promise instead of trusting in the power of the One making the promise, he said, "I will make your descendants as numerous as all the grains of sand in the world. If anyone can number the grains of sand in the world, your descendants too will be numbered." No doubt the promise went beyond human nature. Not only did he promise to make him a father despite so many impediments but also to extend the gift to such a multitude as to be compared with all the grains of sand in the world, and the multitude to be beyond number, wishing as he did to demonstrate the extent of the remarkable increase by the comparison.

Abraham, styled "the friend," [Isaiah 41:8] was found faithful, inasmuch as he rendered obedience to the words of God. He, in the exercise of obedience, went out from his own country, and from his kindred, and from his father's house, in order that, by forsaking a small territory, and a weak family, and an insignificant house, he might inherit the promises of God. For God said to him, "Get you out from your country, and from your kindred, and from your father's house, into the land which I shall show you. And I will make you a great nation, and will bless you, and make your name great, and you shall be blessed. And I will bless them that bless you, and curse them that curse you; and in you shall all the families of the earth be blessed." [Genesis 12:1-3] And again, on his departing from Lot, God said to him, "Lift up your eyes, and look from the place where you now are, northward, and southward, and eastward, and westward; for all the land which you see, to you will I give it, and to your seed forever. And I will make your seed as the dust of the earth, [so that] if a man can number the dust of the earth, then shall your seed also be numbered." [Genesis 13:14-16] And again [the Scripture] says, "God brought forth Abram, and spoke unto him, Look up now to heaven, and count the stars if you are able to number them; so shall your seed be. And Abram believed God, and it was counted to him for righteousness." [Genesis 15:5-6] On account of his faith and hospitality, a son was given him in his old age; and in the exercise of obedience, he offered him as a sacrifice to God on one of the mountains which He showed him.

Note, The same God that provides the inheritance provides the heirs. He that has prepared the holy land prepares the holy seed; he that gives glory gives grace to make meet for glory.

They were so in Solomon's time, 1 Kings iv, 20. Judah and Israel were many as the land which is by the sea in multitude. This God here gives him the promise of.