Fire symbolizes God’s presence and power. God spoke to Moses out of a burning bush and guided the Israelites with a pillar of fire. In this verse he promises to go before them like a devouring fire, judging the nations that defy him.
Fire can be either helpful or harmful. Contained, it provides light, warmth and fuel. Uncontained, it devours everything in its path. One way or another, God will be a fire in your life. If you defy him, he will burn with justice. If you serve him, he will illuminate the path before you.