Paul confirms his identification of his converts as the children of the Jerusalem above with a quotation from Isaiah 54:1. This prophecy assures Israel during her barren time of the Babylonian captivity that she will one day have more children than ever before. The Jews took it as a prophecy not only of the restoration of Israel but also of the time when multitudes of Gentiles would turn to God and claim Israel as their mother by becoming full members of the Jewish nation. Paul sees the fulfillment of the prophecy in the birth and growth of the church. The multiplication of the children of Sarah and the heavenly Jerusalem was a tangible reality for Paul as he witnessed the faith of Gentiles and their reception of the Spirit. Moreover, they were not born in the ordinary way but as the result of a promise (v. 23)—this promise from Isaiah! As Paul saw this ancient promise of God fulfilled in his own mission to the Gentiles, he must have also fulfilled the commands of the prophecy: Be glad . . . break forth and cry aloud! What a wonderful surprise it was for him to see God fulfilling his word in this way as he preached the gospel to Gentiles.