John Calvin Commentary Isaiah 10:21

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

A remnant shall return. This is a confirmation of the previous statement. Yet in the words שאר ישוב (Shear Yashub), a remnant shall return, there appears to be an allusion to that passage in which Isaiah’s son was called Shear-jashub (Isaiah 7:3). In our observations on it, we stated that this peculiar name was given to him in reference to the event, so that it might be regarded as a pledge of the future deliverance about which his father prophesied. It was necessary that the Jews should be confirmed in various ways, so that they might be convinced that the Lord would eventually bring them back. This is also the design of what he immediately adds—

To the mighty God; that is, to Him whom the people, after returning from their former apostasy, will acknowledge as the guardian of their salvation. This attribute, mighty, is ascribed to God because of the occasion on which the words were used. He might have thought it sufficient to express power by the name אל (El), God, which also signifies mighty; but he also chose to add to it גבור (gibbor), that is, strong or mighty, in order to excite the people to greater confidence. How could the people turn to the Assyrians and Egyptians, unless it was because they did not think that God was sufficient for them? This is the source of all evils, when we are not fully convinced that in God is everything that can be desired for our salvation.