Because galatians 4:21-31
>Tell me, ye that desire to be under the law, do ye not hear the law?
>For it is written, that Abraham had two sons, the one by a bondmaid, the other by a freewoman.
>But he who was of the bondwoman was born after the flesh; but he of the freewoman was by promise.
>Which things are an allegory: for these are the two covenants; the one from the mount Sinai, which gendereth to bondage, which is Agar.
>For this Agar is mount Sinai in Arabia, and answereth to Jerusalem which now is, and is in bondage with her children.
>But Jerusalem which is above is free, which is the mother of us all.
>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.
>Now we, brethren, as Isaac was, are the children of promise.
>But as then he that was born after the flesh persecuted him that was born after the Spirit, even so it is now.
>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.
>So then, brethren, we are not children of the bondwoman, but of the free.