For it is written that Abraham had two sons, one by a slave woman and one by a free woman. But the son of the slave was born according to the flesh, while the son of the free woman was born through promise.

Now this may be interpreted allegorically: these women are two covenants:

1. One is from Mount Sinai, bearing children for slavery; she is Hagar. Now Hagar is Mount Sinai in Arabia; she corresponds to the present Jerusalem, for she is in slavery with her children.
2. But the Jerusalem above is free, and she is our mother. For it is written,

“Rejoice, O barren one who does not bear;
break forth and cry aloud, you who are not in labor!
For the children of the desolate one will be more
than those of the one who has a husband.”

-- Isaiah 54:1

Now you, brothers, like Isaac, are children of promise. But just as at that time he who was born according to the flesh persecuted him who was born according to the Spirit, so also it is now.

But what does the Scripture say?

“Cast out the slave woman and her son, for the son of the slave woman shall not inherit with the son of the free woman.” -- Genesis 21:10

So, brothers, we are not children of the slave but of the free woman.

-- Galatians 4:22-31 (ESV)