John Gill, on Gen 17:21

“But my covenant I will establish with Isaac, whom Sara shall bring forth to thee at this time in the next year.”

“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, and to those only in the line of Jacob, and especially that of the Messiah springing from him, which circumcision had a respect unto: whom Sarah shall bear unto thee, at this set time, in the next year: that is, at the end of nine months, which is the set time a woman goes with child.”

Imported from an open dataset — not yet checked against the printed edition.