Paul recognizes that the Galatians are young in faith and still have much to learn. However, he doesn't want them to go backward. Having claimed this new identity, with its promise of freedom from the sinfulness and brokenness of the world, Paul doesn't want them to give away this identity and its freedom.
This leads to a counter-intuitive understanding of Jewish history, which Paul then presents. As we learn in Genesis, Abraham had two main sons. Ishmael was the son of Abraham's wife's servant; Isaac was the son of Abraham's wife. As such, Ismael represents slavery, while Isaac represents freedom. However, with Paul's understanding of Jewish law as a form of spiritual slavery, he believes that the Gentile Christians must be heirs of Isaac, while the Jews (who are biological descendants of Isaac) are the heirs of Ishmael, blind to the freedom that Jesus Christ offers. (If this seems like an argument where Paul is straining logic and history, many people have found it confounding through the centuries.)
[links to Bible Gateway open in a new window]Reading the Bible in Its Entirety
If you are planning to read the Bible in its entirety, you should read Galatians 4:1-5:1 today.