John Gill Commentary Isaiah 10:21

"A remnant shall return, [even] the remnant of Jacob, unto the mighty God." — Isaiah 10:21 (ASV)

The remnant shall return This is said in allusion to Shearjashub, the name of Isaiah's son, (Isaiah 7:3) which signifies "the remnant shall return", and was imposed on him, to give assurance of it; meaning, either that they should return from the Babylonish captivity, as they did, or to God by repentance; or rather the sense is, they shall turn to the Lord, be converted to Christ, to the faith and obedience of him, as some of them were when he came, a few, not all, only a remnant, as it is explained in the next clause:

[even] the remnant of Jacob, to the mighty God ; the Messiah, so called, (Isaiah 9:6) .

The Targum is, "the remnant which have not sinned, and are turned from sin; the remnant of the house of Jacob shall return to worship before the mighty God."