Albert Barnes Commentary Galatians 4:30

"Howbeit what saith the scripture? Cast out the handmaid and her son: for the son of the handmaid shall not inherit with the son of the freewoman." — Galatians 4:30 (ASV)

Nevertheless. But, (alla.)

What says the Scripture? What does the Scripture teach on the subject? What lesson does it convey regarding the slave?

Cast out the bondwoman and her son. This was the language of Sarah, in an address to Abraham, requesting him to cast out Hagar and Ishmael (Genesis 21:10). That was done. Paul uses it here as applicable to the case before him. As used by him, the meaning is, that everything like servitude in the gospel is to be rejected, as Hagar and Ishmael were driven away.

It does not mean, as it seems to me, that they were to expel the Jewish teachers in Galatia, but that they were to reject everything like servitude and bondage; they were to adhere only to that which was free. Paul cannot mean here that the passage in Genesis 21:10 originally had reference to the gospel, for nothing evidently was further from Sarah's mind than any such reference. Nor can it be shown that he meant to approve of or vindicate Sarah's conduct. But he finds a passage applicable to his purpose, and he conveys his ideas in that language as exactly expressing his meaning. We all use language in that way wherever we find it.