We fear God because his judgment of sin is severe. God enacts his judgment on the basis of his holiness and omniscience. Thus his standards and knowledge of the sinner, individually or collectively, are infallibly true. As Abraham said, “Will not the Judge of all the earth do right?” (Ge 18:25).
In this passage, Isaiah describes a fierce rivalry between the split kingdoms of old Israel. The northern king with his Syrian ally was attacking Jerusalem, capital of the south (see Isa 7:1–2). The prophet assures Judah that Jerusalem will survive as Assyria’s army crushes this alliance (see Isa 8:14–17).
Yet even wicked Assyria with its powerful army would ultimately be punished by God (see Isa 10:12). Isaiah declares that God in his holiness will be victorious, saving his repentant people. He is “the Mighty God” (Isa 10:20–21), who is to be feared.