What does Isaiah 10:17 mean?
The kings of Assyria may have thought of themselves as spreading "the light" of the Assyrian Empire around the world (Isaiah 10:13–14). However, the Lord has declared that He will punish the arrogant speech of the Assyrian king. Once the Lord has finished using Assyria to judge His people, He will end the boastful look in the king's eyes (Isaiah 10:16).The Lord now refers to Himself as the light of Israel, which will become a fire. There will be no more need for the destructive tool of Assyria. The Lord will burn and devour the Assyrian king's "thorns and briers" in a single day.
Isaiah is using the same word picture he painted at the end of the previous chapter (Isaiah 9). In that chapter he described Israel's wickedness as fire burning through the briers and thorns, and then leaping up to consume the forest (Isaiah 9:18–19). In that case, the Lord's wrath against His people took the form of the Assyrian armies. Now it is those armies who will be consumed.