What does Galatians 4:28 mean?
Paul continues his allegory about the difference between living under the law, versus being made right with God through faith in Christ. It is a difference between slavery and freedom. He has connected those under the law as being born into slavery, in the same way that those born to a slave woman—like Abraham's slave-wife Hagar (Genesis 16:1–3)—are born into slavery.In comparison, those born to a free woman like Abraham's wife Sarah (Genesis 21:1–3) are born into freedom. In that way, those who have trusted in Christ, as the Galatian Christians had initially done, are like Isaac. They are born into freedom. More specifically, Christians are the children of promise, as Isaac was.
In fact, Christians become children of God by the very same promise God gave to Abraham and fulfilled in Isaac. Earlier in Galatians, Paul has written that "those who are of faith are blessed along with Abraham" (Galatians 3:9) and that "if you are Christ's, then you are Abraham's offspring, heirs according to promise" (Galatians 3:29). In that sense, Christians are truly Isaac's siblings.