Verses 15-16 (Genesis 16:15-16)

It is here taken for granted, though not expressly recorded, that Hagar did as the angel commanded her, returning to her mistress and submitting herself; and then, in the fulness of time, she brought forth her son. Note, Those who obey divine precepts shall have the comfort of divine promises. This was the son of the bond-woman that was born after the flesh (Gal. 4:23), representing the unbelieving Jews, Gal. 4:25. Note, 1. Many who can call Abraham father are 608 yet born after the flesh, Matt. 3:9. 2. The carnal seed in the church are sooner brought forth than the spiritual. It is an easier thing to persuade men to assume the form of godliness than to submit to the power of godliness.

- Matthew Henry's Complete Commentary