9 “God shall reign over all nations”. Who reigned over one nation, “shall reign” (says He) “over all nations.” When this was said, God reigned over one nation. It was a prophecy, the thing was not yet shown. Thanks be to God, we now see fulfilled what before was prophesied. A written promise God sent unto us before the time, the time fulfilled He has repaid us. “God shall reign over all nations,” is a promise. “God sits upon His Holy Seat.” What then was promised to come, now being fulfilled, is acknowledged and held.
“God sits upon His Holy Seat.” What is His Holy Seat? Haply says one, The Heavens, and he understands well. For Christ has gone up, as we know, with the Body, wherein He was crucified, and sits at the right hand of the Father; thence we expect Him to come to judge the quick and the dead. “God sits upon His Holy Seat.” The Heavens are His Holy Seat. Will you also be His Seat? Think not that you can not be; prepare for Him a place in your heart. He comes, and willingly sits. The same Christ is surely “the Power of God, and the Wisdom of God:” and what says the Scripture of Wisdom Herself?
The soul of the righteous is the seat of Wisdom. If then the soul of the righteous is the seat of Wisdom, be your soul righteous, and you shall be a royal seat of Wisdom. And truly, brethren, all men who live well, who act well, converse in godly charity, does not God sit in them, and Himself command? Your soul obeys God sitting in it, and itself commands the members. For your soul commands your members, that so may move the foot, the hand, the eye, the ear, and itself commands the members as its servants, but yet itself serves its Lord sitting within. It cannot well rule its inferior, unless its superior it have not disdained to serve.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)