This is allegorically speaking, for these women are two covenants: one proceeding from Mount Sinai bearing children who are to be slaves; she is Hagar. Now this Hagar is Mount Sinai in Arabia and corresponds to the present Jerusalem, for she is in slavery with her children. But the Jerusalem above is free; she is our mother. [Galatians 4:24-26]
Paul was using Hagar and Sarah as examples to the Galatians to illustrate his point about the 2 covenants: law and grace. In the beginning, he had already stated he was using them as metaphors. Hagar represented the earthly Jerusalem and was the mother of those born under law. Hagar’s children were slaves to the law. Sarah represented the heavenly Jerusalem and was the mother of those born under grace. Sarah’s children were free from the law.