Galatians 4:22

For it is written that Abraham had two sons, one by the handmaid, and one by the freewoman.

Ishmael was the son of Hagar, Sarah's maid, whom she gave to Abraham, in order to claim a son (by such a device) for herself. Abraham had many sons by concubines, but they were his property, not Sarah's. Isaac was the actual son born to Sarah, born as a result of the promise of God long after the time when either Abraham or Sarah might have expected to have children. Sarah of course was free, the lawful wife of the mighty patriarch. The full account of all this is in Genesis, much of the entire book being given over to the recounting of it. A summary of the allegory Paul was about to give is the following:

<LINES><MONO>

JUDAISM CHRISTIANITY

The bondwoman, Hagar The freewoman, Sarah

Son of the bondwoman, Ishmael Son of the freewoman, Isaac

Natural birth Supernatural birth by promise

Mount Sinai, the Law Mount Zion, the Law of Christ

The earthly Jerusalem The heavenly Jerusalem

Enslaved Free

Fruitful Barren (at first)

Small offspring Large offspring

Persecuting Persecuted

Expulsion Inheritance

Judaism a bondage Christians freeMONO>LINES>

These analogies will clarify many of the points Paul made in the next few verses.