Psalm 99:1 The Lord reigneth, let the people tremble: he sitteth between the cerubims, let the earth be moved.
The Lord is an awesome and terrible God, who rules mightily over everything. Therefore, the people respond with trembling and the earth itself is moved at His glorious reign. Trembling and fear before a mighty and holy God is the natural human response as sinners before a holy God. Thunder and lightening, wind and hail, are only minuscule expressions of the mighty power of holiness directed against sin and evil. Like close by thunder causes us to tremble, so does the fearsome presence of God.
The Mighty God is present on the earth between the Angels on the Ark of the Covenant in the Holy of Holies in the temple. That very fact causes the people to tremble and the earth to shake. And every natural disaster reminds us of the holy judgment of God against Sin.
Whenever we think of God, we too, react with fear and trembling before a holy God, but the next thought of God sees the overpowering love of God pouring out His holy wrath upon His Son on the cross. We react with worship and loving thanks and praise for the life-giving power of love and mercy. We tremble twice: first, when we hear the words of God’s Law, and second, when we hear the words of God’s Gospel. We tremble over God’s overpowering Holiness and over God’s overwhelming Love. We look at the Cross, and it causes us to tremble, tremble, tremble.