Habakkuk compares the awe and trembling of the righteous man to the awe of nature before God. The mountains, the rivers and the sea, the sun the moon and the sky, all tremble before God. But they are not the recipients of God's wrath. Only the evil nations and the tormentors of the poor are the recipients of God's anger. God comes to destroy evil and liberate the righteous. God's weapons are perfect and exact. So yes we may tremble as do the mountains, but we can still rejoice when we are afraid, because it is God who is coming to save us.