Paul’s quotation emphasizes the fruit and fertility of the barren woman, who symbolizes Israel/Jerusalem. There was a time when she had no/few children (a reference to the time of Israel’s exile). However, there will come a time where there will be many children, after the return from exile, more children even than there were before.1 Paul is claiming that this time has now arrived. The prophecy of Isaiah is being fulfilled, not in an earthly city, but in a spiritual one that spreads across the globe.2 Specifically, this means the growth of the church through Gentile believers.3 Since the church includes both Jew and Gentile, it is more fruitful than Mosaic law (Hagar, present Jerusalem) which includes Jews only.4
27 For it is written, “Rejoice, O barren one who does not bear; break forth and cry aloud, you who are not in labor! For the children of the desolate one will be more than those of the one who has a husband.”