Pastor reminds us to "think like Paul", who summarizes the key purpose of this letter: "We are being delivered from this present Evil Age into the Age which is to come" (Galatians 1:3-4) by faith in Christ Jesus, not by the Law. Before faith in Christ came, we were held captive under the Law, imprisoned until the coming of faith. The Law could not give life. Faith in Christ Jesus gives life.
Paul goes on to use an allegory [44:30] to contrast the Abrahamic and Sinai covenants using two women, Hagar and Sarah and their sons to these two covenants. Hagar and Ishmael are Mt. Sinai, representing the law. Sarah and Isaac are Mt. Zion, the heavenly Jerusalem, representing the gospel.
This is a picture contrasting slaves and freemen. Pastor skillfully uses two stacks upon which he lays images to describe each. We are to realize where we are and where the bad guys are and cast out the slave woman.