John Calvin Commentary Psalms 99:1

"Jehovah reigneth; let the peoples tremble: He sitteth [above] the cherubim; let the earth be moved." — Psalms 99:1 (ASV)

Jehovah reigns. The people, who were previously called upon to rejoice, are now commanded to tremble. For since the Jews were surrounded by enemies, it was extremely important that God’s power should be exalted among them, so that they might know that, while under His protection, they would be always and entirely safe from the hatred and fury of every enemy.

The Hebrew word רגז, ragaz, as we have seen elsewhere, sometimes signifies to tremble, at other times, to be angry, and, in short, denotes any strong emotion arising either from anger or fear. Accordingly, the prophet here intends that God, in the deliverance of His chosen people, should give such a tangible display of His power that it would strike all the nations with dismay, and make them realize how recklessly they had rushed toward their own destruction.

For it is particularly in relation to humankind that God is said to reign, when He exalts Himself by the magnificent displays of His power. This is because, while the help He gives them remains unseen, unbelievers act more presumptuously, as if there were no God.