Matthew Poole and continuators on Galatians 4:23

23 But he who was of the bond-woman was born after the flesh , but he of the free-woman was by promise t:

They were both (in a sence) born after the Flesh, viz. in a natural way and course of generation: but after the flesh is plainly in this verse opposed to by promise: and the meaning is, that Ishmael, the son of Hagar, was not that son of Abraham to whom the promise was made, that in him all the Nations of the Earth should be blessed, Genesis 15.4. chap. 17.19. Isaac is said to have been born after the promise, either because God gave Isaac to Abraham, in complement, or fulfilling of the promise made to him, that he should have an Heir out of his own loins; or because the mighty and miraculous power of God was seen in his production, inabling Abraham at those years to beget, and Sarah to bear a child, when both their bodies were as dead.

Marginal references: Rom. 9.7, 8.