Paul wants Scripture, not his own opinion, to settle the matter. He opened this section by asking those who wanted to be under the law, "do ye not hear the law?" (Galatians 4:21), so he now lets the law's own book give the verdict. Barnes puts the question in plain words: "What does the Scripture teach on the subject?" Gill notes that "what saith the scripture?" was a Talmudic form of citing Scripture, a way Jewish teachers introduced a proof text, so Paul is answering his opponents in a form they would recognize. Paul has just admitted that the persecution is still going on (Galatians 4:29). The question turns the readers from what is happening to them toward what God has said about who the heir is.

Paul answers his own question at once with words from Genesis: "Cast out the bondwoman and her son: for the son of the bondwoman shall not be heir with the son of the freewoman" (Galatians 4:30). Then he draws the conclusion: "So then, brethren, we are not children of the bondwoman, but of the free" (Galatians 4:31). He follows it with a command: "Stand fast therefore in the liberty wherewith Christ hath made us free, and be not entangled again with the yoke of bondage" (Galatians 5:1).

The quoted words were first spoken by Sarah. At the feast for Isaac's weaning, she saw Ishmael, "the son of Hagar the Egyptian," mocking, and told Abraham, "Cast out this bondwoman and her son" (Genesis 21:8-10). Abraham was grieved, but God told him, "in all that Sarah hath said unto thee, hearken unto her voice; for in Isaac shall thy seed be called" (Genesis 21:11-12). That is why Paul can call Sarah's words "the scripture": Gill says they were "approved of and confirmed by God." Paul shapes the ending to fit his point. Genesis 21:10 says "with my son, even with Isaac"; Paul writes "with the son of the freewoman."

In God's family, the inheritance comes by promise, not by birth after the flesh or by keeping the law. Paul has already said that "if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise" (Galatians 3:29). This question lets Scripture confirm it: the son of the bondwoman "shall not be heir." God's way of giving the inheritance cannot be mixed with a system of earning it.

Commentators apply the casting out differently. Gill sees in it a type of the rejection of those who seek righteousness by the law from the gospel church. Barnes is careful to say that, as it seems to him, it does not mean the Galatians should expel the Jewish teachers among them; rather, "everything like servitude in the gospel is to be rejected." The difference matters. Barnes keeps the command pointed at a way of relating to God, not at people to be driven out, and Paul's own conclusion is about who believers are (Galatians 4:31).