For this is the word of promise, At this time will I come, and Sara shall have a son. For this is the word of promise, etc. - That is, this is evidently implied in the promise recorded Genesis 18:10 : At this time I will come, saith God, and exert my Divine power, and Sarah, though fourscore and ten years old, shall have a son; which shows that it is the sovereign will and act of God alone, which singles out and constitutes the peculiar seed that was to inherit the promise made to Abraham.
It should be considered that the apostle, in this and the following quotations, does not give us the whole of the text which he intends should be taken into his argument, but only a hint or reference to the passages to which they belong; directing us to recollect or peruse the whole passage, and there view and judge of the argument.
That he is so to be understood appears from the conclusion he draws, Romans 9:16 : So then, it is not of him that willeth, nor of him that runneth, but of God that showeth mercy. In his arguments, Romans 9:7, Romans 9:8, etc., he says not one word of Abraham's willing Ishmael to be the seed in whom the promise might be fulfilled; nor of Isaac's willing Esau; nor of Moses' willing and interceding that the Israelites might be spared; nor of Esau's running for venison; but by introducing these particulars into his conclusion, he gives us to understand that his quotations are to be taken in connection with the whole story, of which they are a part; and without this the apostle's meaning cannot be apprehended.
The same may be said of his conclusion, Romans 9:18 : Whom he will he hardeneth: hardeneth is not in his argument, but it is in the conclusion. Therefore hardening is understood in the argument, and he evidently refers to the case of Pharaoh. The generality of the Jews were well acquainted with the Scripture, and a hint was sufficient to revive the memory of a whole passage. - Taylor, p. 330.