Charles Spurgeon 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 of the freewoman was by promise.

In the power of God, born after both father and mother had ceased to be capable of becoming parents, born in the power of God.