Thematic Bible: Isaac born to

So Sarah became pregnant and bore Abraham a son in his old age at the appointed time that God had told him. Abraham named his son -- whom Sarah bore to him -- Isaac.

For it is written that Abraham had two sons, one by the slave woman and the other by the free woman. But one, the son by the slave woman, was born by natural descent, while the other, the son by the free woman, was born through the promise. These things may be treated as an allegory, for these women represent two covenants. One is from Mount Sinai bearing children for slavery; this is Hagar. read more.
Now Hagar represents Mount Sinai in Arabia and corresponds to the present Jerusalem, for she is in slavery with her children. But the Jerusalem above is free, and she is our mother. For it is written: "Rejoice, O barren woman who does not bear children; break forth and shout, you who have no birth pains, because the children of the desolate woman are more numerous than those of the woman who has a husband." But you, brothers and sisters, are children of the promise like Isaac. But just as at that time the one born by natural descent persecuted the one born according to the Spirit, so it is now. But what does the scripture say? "Throw out the slave woman and her son, for the son of the slave woman will not share the inheritance with the son" of the free woman.