Albert Barnes Commentary Psalms 47:8

"God reigneth over the nations: God sitteth upon his holy throne." — Psalms 47:8 (ASV)

God reigneth over the heathen - Over the “nations;” not over the “heathen” in the sense in which that term is understood now. It does not mean that God reigns, or that he has set up his throne over the people that have not the true religion, but that he is exalted over the “nations” of the earth as such; or, that he has universal dominion. See the notes at Psalms 46:10.

God sitteth upon the throne of his holiness - Upon his holy throne. The idea is that his government is established in holiness or justice.