John Calvin Commentary Genesis 15:4

"And, behold, the word of Jehovah came unto him, saying, This man shall not be thine heir; But he that shall come forth out of thine own bowels shall be thine heir." — Genesis 15:4 (ASV)

This shall not be your heir. From this we infer that God had approved Abram's wish. From which also follows the other point: Abram had not been driven by any fleshly motive to offer up this prayer, but by a pious and holy desire to enjoy the blessing promised to him.

For God not only promises him a seed, but a great people, whose number would equal the stars of heaven. Those who interpret the passage allegorically, implying that a heavenly seed was promised to him that could be compared with the stars, may hold their own opinion; but we maintain what is more sound: namely, that Abram's faith was increased by the sight of the stars.

For the Lord, to affect His own people more deeply and to penetrate their minds more effectively, after He has reached their ears here by His word, also captures their eyes with external symbols, so that eyes and ears might be in agreement.

Therefore, the sight of the stars was not unnecessary; God intended to impress this thought upon Abram's mind: ‘He who by His word alone suddenly produced such a numerous host with which He might adorn the previously vast and desolate heaven—will He not be able to replenish my desolate house with offspring?’

It is not necessary, however, to imagine a nocturnal vision, just because the stars, which are hidden from our sight during the day, would then have appeared. For since the whole occurred in a vision, Abram had a wonderful scene set before him, which would clearly reveal hidden things to him.

Therefore, though he perhaps did not move a step, it was still possible for him to be led out of his tent in the vision.

The question now arises concerning which seed this promise refers to. It is certain that neither the posterity of Ishmael nor of Esau is to be included in this account, because the legitimate seed is to be determined by the promise, which God determined should remain in Isaac and Jacob. Yet the same doubt arises regarding 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, in a general sense, extended to the whole people whom God has adopted for Himself. But since many were alienated by their unbelief, we must look to Christ, who alone distinguishes true and genuine sons from those who are illegitimate. By pursuing this method, we find Abram's posterity reduced to a small number, so that afterwards it may be increased all the more. 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. More will be said on this point in the seventeenth chapter (Genesis 17:1).