Galatians 4
First, the apostle pointed to two covenants. One, the Mosaic, had its origin at Mount Sinai. Those under this legal covenant were slaves. As Hagar brought forth a slave, so does the Law. At this point the reader is expected to understand and supply the implicit reference to the Abrahamic Covenant, a gracious system represented by Sarah which through its messianic promise brought forth children who are free.
4:25–26. Next, Paul pointed to two Jerusalems. Hagar also stood for the first-century city of Jerusalem, a city enslaved to Rome and in slavery to the Law. Sarah, on the other hand, corresponded to the Jerusalem … above, the mother of all the children of grace. This heavenly city, which one day will come to earth (cf. Rev. 21:2), is now the “city of the living God” (cf. Heb. 12:22), the home of departed believers of all ages.
4:27. The quotation from Isaiah 54:1 prophesied the changing fortunes of Israel, which Paul applied to Sarah’s history. Israel before her Babylonian Captivity was likened to a woman with a husband. The barren woman was Israel in Captivity. The woman bearing more … children may have pictured Israel restored to the land after the Exile, but more particularly it portrays her millennial blessings. Paul applied this passage (he did not claim it was fulfilled) in this context to Sarah, who though previously barren, was later blessed with a child, and who would ultimately enjoy a greater progeny than Hagar.