Sing, O barren, thou that didst not bear; break forth into singing, and cry aloud, thou that didst not travail with child: for more are the children of the desolate than the children of the married wife, saith the LORD.
Israel converted is compared to a wife (Isaiah 54:5; Isaiah 62:5) put away for unfaithfulness, but now forgiven and taken home again. The converted Gentiles are represented as a new progeny of the long-forsaken but now restored wife. The preeminence of the Hebrew Church as the mother Church of Christendom is the leading idea: the conversion of the Gentiles is mentioned only as part of her felicity (Horsley).
Sing - for joy (Zephaniah 3:14).
O barren - the Jewish Church, once forsaken by God, and therefore during that time destitute of spiritual O barren - the Jewish Church, once forsaken by God, and therefore during that time destitute of spiritual children (Isaiah 54:6).
Thou (that) didst not bear - during the Babylonian exile primarily. Secondarily, and chiefly, during Israel's present dispersion.
More (are) the children of the desolate - the Gentiles adopted by special grace into the original Church (Isaiah 54:3; Isaiah 49:20-21).
Than the children of the married wife - than were her spiritual children, when Israel was still a married wife (under the law, before the Babylonian exile), before God put her away (Maurer). So Paul contrasts the "Jerusalem which is above ... the mother of us all," the universal Church of the New Testament, with the narrow Church of the Old Testament legal dispensation, quoting this very passage (Galatians 4:27). But the full accomplishment of it is yet future.