John Calvin Commentary Genesis 22:17

"that in blessing I will bless thee, and in multiplying I will multiply thy seed as the stars of the heavens, and as the sand which is upon the seashore. And thy seed shall possess the gate of his enemies." — Genesis 22:17 (ASV)

Thy seed shall possess the gate of his enemies. He means that the offspring of Abraham should be victorious over their enemies, for in the gates were their bulwarks, and in them they administered judgment. Now, although God often allowed the enemies of the Jews to rule tyrannically over them, yet He so moderated their revenge that this promise always prevailed in the end.

Moreover, we must remember what Paul has previously stated concerning the unity of the seed, for from this we infer that the victory is promised not indiscriminately to the sons of Abraham, but to Christ and to His members, insofar as they adhere together under one Head. For unless we retain some mark that distinguishes between the legitimate and the degenerate sons of Abraham, this promise will indiscriminately include the Ishmaelites and Idumeans as well as the people of Israel. But the unity of a people depends on its head. Therefore, the prophets, whenever they wish to confirm this promise of God, assume the principle that those who have previously been divided will be united under David in one body.

More on this subject may be found in Genesis 12:1.