What does the Bible say about the allegory of Hagar and Sarah?

In Galatians 4, the apostle Paul explains that Hagar represents the old covenant of law and works, while Sarah represents the new covenant of grace. This allegory highlights the truth that those born of grace, like Isaac, are the true children of God, while those who rely on their own efforts, represented by Ishmael, are excluded from the inheritance. This duality emphasizes the core Reformed belief that salvation is solely by God's grace and not influenced by human works or conditions.