In the early church one of the greatest enemies of the gospel was legalism, the belief and the attitude that we are made acceptable to God on the basis of our ability to keep the law. A similar belief is called nomism, which is the belief that we believers must place themselves under the law as a supervisor to govern our lives. When this happens, the church becomes proud and self-sufficient, losing its freedom and joy in Christ.
Paul used the story of Abraham, Sarah and Hagar to illustrate the futility and slavery of living by the law, by what we can do for God, and the freedom of living by faith in what Jesus Christ has done and is doing for us. In this story, Ishmael, the son of the slave, and Isaac, the son of the free woman, would inherit the position of their mothers.
So it is today. Who is your mother? Are you a child of Hagar or a child of Sarah? Your freedom depends on your answer!