What does Isaiah 8:13 mean?

Isaiah quoted the Lord's command not to fear what the people of Judah fear (Isaiah 8:12). This was not because the people had nothing to fear. The people were right to be afraid. They were wrong about where their fear was directed. What they feared was only a small effect; the real cause was God Himself.

Isaiah has already described the terrible times to come for Judah (Isaiah 8:8). They will be threatened and defeated by powerful enemies. Still, the people don't realize that it is the Lord, their own God, who will bring the destruction upon them. Instead of seeing the Assyrians as all-powerful, the Lord tells Isaiah to honor Him as holy. The Lord is truly the source of all power. The Assyrians are a blip in history which the Lord could easily wipe out. The only possibility of salvation from destruction comes through honoring the Lord, the true source of that destruction, above everything else in life.

Now the Lord tells Isaiah to fear and dread only God Himself. In the biblical context, "fear" of God means something other than blind panic or terror. It involves the idea of respect, honor, obedience, and reverence. At the same time, it does include a healthy sense of "fear." Just as people "fear" fire and its destructive potential without living in constant alarm, one can properly "fear" God without experiencing fright. We "fear" what fire can do when it's misused; we should "fear" the consequences of defying our Creator.

Isaiah is to revere God, not worry about the fears expressed by others in Judah. Judgment will come on Judah from the Lord. He is the one they should focus their fear on. And He is the one they should return to in repentance, obedience, and genuine worship.