Paul compares two mothers, two sons, and two covenants to show that those who are in Christ are children of God, while those who are under the law are slaves with no place in the family and no share in the inheritance. If the gospel makes us sons and daughters of God; if in the gospel God adopts us into his family by his grace and gives us a share in all the rights and privileges of the children of God; if the gospel assures us that God is our heavenly Father who loves us; why then would we want to turn back from grace and go back to living under law?