Jamieson, Fausset and Brown on Isaiah 1:27

27. redeemed--temporarily, civilly, and morally; type of the spiritual redemption by the price of Jesus Christ's blood (1Pe 1:18, 19), the foundation of "judgment" and "righteousness," and so of pardon. The judgment and righteousness are God's first (Is 42:21; Ro 3:26); so they become man's when "converted" (Ro 8:3, 4); typified in the display of God's "justice," then exhibited in delivering His covenant-people, whereby justice or "righteousness" was produced in them.

converts--so Maurer. But Margin, "they that return of her," namely the remnant that return from captivity. However, as Isaiah had not yet expressly foretold the Babylonian captivity, the English Version is better.