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.