Charles Ellicott Commentary Galatians 4:22

"For it is written, that Abraham had two sons, one by the handmaid, and one by the freewoman." — Galatians 4:22 (ASV)

For.—This particle would naturally not be expressed in English. It was a reason for the question which had been asked just before: “For the Law does supply a case in point.”

The one by a bondmaid.—Hagar, it seems from Genesis 16:1, was an Egyptian. The word for “bondmaid” was not confined to this sense in earlier Greek, but was used for any young girl.