What does the Bible mean by casting out the bondwoman and her son?

In Galatians 4:30, when Paul speaks of casting out the bondwoman and her son, he references the necessity of rejecting a reliance on the law for justification. Hagar represents the old covenant of law, whereas Sarah represents the new covenant of grace. This allegorical teaching asserts that believers should dismiss any notion that mixes law with grace, as true salvation can only come through complete reliance on God's promise fulfilled in Christ. This serves as a reminder of the stark division between those who adhere to legalism and those who trust in the grace of God.