What does Galatians 4:23 mean?

The Galatian Christians are being deceived by a group known as the Judaizers. These false teachers claim that one must follow the law of Moses, in addition to believing in Christ, in order to be saved (Galatians 2:4). In this passage, Paul continues building an illustration based on the two sons of Abraham, Ishmael and Isaac. He will compare their legal standing to slavery under the law and freedom in Christ.

In spite of the fact that God had promised a child to Abraham and his wife Sarah (Genesis 15:3–6), they eventually chose to have a child through Sarah's Egyptian slave-girl, instead (Genesis 16:1–2). This succeeded, and Ishmael was born, but he was not the child of God's promise. Paul describes him as being born "according to the flesh." Eventually, Sarah did give birth to a son—when she was 90 (Genesis 21:1–3)! This boy, Isaac, was indeed the child of the promise. He was the long-awaited one. Once he was born, Ishmael's status became even less significant.

Paul will continue by explaining that this echoes the difference between seeking salvation through works, as opposed to accepting it through faith in Christ.