For Sarah conceived, and bare Abraham a son in his old age, at the set time of which God had spoken to him. (21:2)

And Abraham called the name of his son that was born unto him, whom Sarah bare to him, Isaac. (21:3)

--Galations--

For it is written, that Abraham had two sons, the one by a bondmaid, the other by a freewoman. (4:22)

But he [who was] of the bondwoman was born after the flesh; but he of the freewoman [was] by promise. (4:23)

Which things are an allegory: for these are the two covenants; the one from the mount Sinai, which gendereth to bondage, which is Agar. (4:24)

For this Agar is mount Sinai in Arabia, and answereth to Jerusalem which now is, and is in bondage with her children. (4:25)

But Jerusalem which is above is free, which is the mother of us all. (4:26)

For it is written, Rejoice, [thou] barren that bearest not; break forth and cry, thou that travailest not: for the desolate hath many more children than she which hath an husband. (4:27)

Now we, brethren, as Isaac was, are the children of promise. (4:28)

But as then he that was born after the flesh persecuted him [that was born] after the Spirit, even so [it is] now. (4:29)

Nevertheless what saith the scripture? Cast out the bondwoman and her son: for the son of the bondwoman shall not be heir with the son of the freewoman. (4:30)