Abraham had two sons. Ishmael and Isaac. Abraham subsequently had several sons by Keturah, after the death of Sarah, Ge 25:1-6. But the two sons by Hagar and Sarah were the most prominent, and the events of their lives furnished the particular illustration which Paul desired.

The one by a bond-maid. Ishmael, the son of Hagar. Hagar was an Egyptian slave, whom Sarah gave to Abraham in order that he might not be wholly without posterity, Ge 16:3.

The other by a free-woman. Isaac, the son of Sarah, Ge 21:1,2.

{c} "one by a bond-maid" Ge 16:15 {d} "the other" Ge 21:1,2