The quoted passage, Isaiah 54:1, promised Israel they would thrive after the exile to Babylon. Those who have faith in Jesus Christ are Sarah’s children (Galatians 4:26). This fulfills the promise that Sarah, who was childless, would have more children than Hagar, the slave woman. To the Judaizers (those who insisted on following Jewish law), people not following the law might seem desolate (spiritually barren or unfruitful), but through faith, they become abundantly fruitful as the bride of Christ (2 Corinthians 11:2; Ephesians 5:31–33).