Your Momma is either Hagar or Sarah. It’s one or the other. Paul therefore exhorts the Galatians to cast out one and cling to the other. He says in Galatians 4:30-31. “But what does the Scripture say? ‘Cast out the slave woman and her son, for the son of the slave woman shall not inherit with the son of the free woman.’ So, brothers, we are not children of the slave but of 
the free woman.” You cannot be saved by Grace and saved by works. You can’t even be saved by Grace and works. Obviously, you are either the child of the slave woman or a child of the free woman. It is one or the other. Gromacki explains this well. He writes, “It is impossible to be born of two mothers. The heir could not be born of both Sarah and Hagar. Even so, spiritual heirs cannot be begotten out of grace and out of works at the same time. The Judaizers claimed that a person had to be saved by both faith and works of legalism. In essence, that concept is impossible. That view actually reduces to salvation by works alone.”
Grace and legalism cannot co-exist at the same time. The very idea of the one excludes the other. Either we please God by what we do, or we appropriate what Christ did to please God. It is either one or the other, not both. We see clearly in Paul’s address to the Galatians and throughout history that legalists do not like grace-oriented people because grace humbles the believer. Richison says, “Legalism points to self and self-righteousness. Grace points to the finished work of Christ. One is self-effort the other is Christ-effort. The two ideas are diametrically opposed. We have a tendency to confuse the two.” Oswald Hoffman illustrates what is happening in Galatia through a helpful story about his dog Mack. Lawson tells that story in his commentary: “One night, Hoffman and his wife were sitting outside when Mack began barking loudly at them. They realized what had happened. Mack was off his chain, but he didn’t know it yet. He would run to the spot where the chain usually yanked him back. Then he’d stop, turn, and run in the other direction. As they watched, it took him over ten minutes to realize he was free. Until then, he remained in bondage, even though he had been set free.”
Legalism always results in slavery. Christ redeemed us from the curse of the law. When we go back to the law we diminish Christ’s death for our sin. Paul knew that both the Galatians and he had been born into the family of God in the right way, by grace through faith in Jesus Christ. Like Isaac, they would receive the full inheritance of their spiritual Father through His promises to Father Abraham. But the way we live out our faith daily is more a matter of who our mother is. We can live as if we’re children of Hagar, slaves, or we can live as if we’re children of Sarah, free!