They were both (in a sense) 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: see Genesis 15:4, Genesis 17:19. Isaac is said to have been born after the promise, either because God gave Isaac to Abraham, in completion 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, enabling Abraham at those years to beget, and Sarah to bear, a child, when both their bodies were as dead.
Related Commentaries of Galatians 4:23
Galatians 4:23
23 But he who was of the bondwoman was born after the flesh; but he of the freewoman was by promise.