John Calvin Commentary Galatians 4:23

"Howbeit the [son] by the handmaid is born after the flesh; but the [son] by the freewoman [is born] through promise." — Galatians 4:23 (ASV)

But he who was of the bond woman.

Both were sons of Abraham according to the flesh; but in Isaac there was this peculiarity, that he had the promise of grace. In Ishmael there was nothing besides nature; in Isaac there was the election of God, signified in part by the manner of his birth, which was not in the ordinary course, but miraculous.

Yet there is an indirect reference to the calling of the Gentiles, and the rejection of the Jews: for the latter boast of their ancestry, while the former, without any human interference, have become the spiritual offspring of Abraham.