Genesis 15:4

4. This shall not be thine heir. We hence infer that God had approved the wish of Abram. Whence also follows the other point, that Abram had not been impelled by any carnal affection to offer up this prayer, but by a pious and holy desire of enjoying the benediction promised to him. For God not only promises him a seed, but a great people, who in number should equal the stars of heaven. They who expound the passage allegorically; implying that a heavenly seed was promised him which might be compared with the stars, may enjoy their own opinion: but we maintain what is more solid; namely, that the faith of Abram was increased by the sight of the stars. For the Lord, in order more deeply to affect his own people, and more efficaciously to penetrate their minds, after he here reached their ears by his word, also arrests their eyes by external symbols, that eyes and ears may consent together. Therefore the sight of the stars was not superfluous; but God intended to strike the mind of Abram with this thought, ‘He who by his word alone suddenly produced a host so numerous by which he might adorn the previously vast and desolate heaven; shall not He be able to replenish my desolate house with offspring?’ It is, however, not necessary to imagine a nocturnal vision, because the stars, which, during the day, escape our sight, would then appear; for since the whole was transacted in vision, Abram had a wonderful scene set before him, which would manifestly reveal hidden things to him. Therefore though he perhaps might not move a step, it was yet possible for him in vision to be led forth out of his tent. The question now occurs, concerning what seed the promise is to be understood. And it is certain that neither the posterity of Ishmael nor of Esau is to be taken into this account, because the legitimate seed is to be reckoned by the promise, which God determined should remain in Isaac and Jacob; yet the same doubt arises respecting the posterity of Jacob, because many who could trace their descent from him, according to the flesh, cut themselves off, as degenerate sons and aliens, from the faith of their fathers. I answer, that this term seed is, indiscriminately, extended to the whole people whole God has adopted to himself. But since many were alienated by their unbelief, we must come for information to Christ, who alone distinguishes true and genuine sons from such as are illegitimate. By pursuing this method, we find the posterity of Abram reduced to a small numbers that afterwards it may be the more increased. For in Christ the Gentiles also are gathered together, and are by faith ingrafted into the body of Abram, so as to have a place among his legitimate sons. Concerning which point more will be said in the seventeenth chapter Genesis 17:1