But I will establish My covenant with Isaac, whom Sarah will bear to you at this time next year.”
Here is, I. The promise made to Abraham of a son by Sarai, that son in whom the promise made to him should be fulfilled, that he should be the father of many nations; for she also shall be a mother of nations, and kings of people shall be of her, Gen 17:16. Note, 1. God reveals the purposes of his good-will to his people by degrees.
Commenting on Genesis 17:15-22
But my covenant will one establish with Isaac,.... The covenant of circumcision; for though Ishmael was circumcised, and his posterity practised that rite, yet it was not enjoined them of God; nor was it to them, or served the same purpose as to the Israelites; and particularly the promise of the land of Canaan, made in that covenant, belonged only to the posterity of Isaac...
My covenant will I establish with Isaac - All temporal good things are promised to Ishmael and his posterity, but the establishment of the Lord's covenant is to be with Isaac. Hence it is fully evident that this covenant referred chiefly to spiritual things - to the Messiah, and the salvation which should be brought to both Jews and Gentiles by his incarnation, death, and glorification.