Matthew Poole and continuators on Psalms 50:6

6. And the heavens shall declare his righteousness : for God is judg himself, Selah .

Which they were called to Witness, v. 4. So was the Earth also. But here he mentions the Heavens onely, as I humbly conceive, because they were the most impartial and Considerable Witness in the Case; for men upon Earth might be false Witnesses, either through Ignorance and Mistake, or through Prejudice and Partiality, and Passion: But the Angels understand things more thoroughly and certainly, and are so exactly pure and sinless, that they neither can nor will tell a lye for God; and therefore their Testimony is more valuable. Or the meaning is, that God would Convince the People of his Righteousness, and of their own Wickedness, by terrible Thunders and Lightnings, and Storms or other dreadful Signs wrought by him, in the Heaven or the Air: by which he did Convince his People in two like Cases, Deut. 5. 22, 23. &c. 1 Sam. 12. 17, 18, 19.

In his own Person, or immediately. God will not now reprove them, or contend with them by his Priests or Prophets, with whom they may easily strive, as they used to do, but he will do it in an immediate and extraordinary manner from Heaven; and therefore they shall be forced to acknowledge his Righteousness, and their own unrighteousness; as they must needs do, when the Contest is between them, and that God who is the great Judge of the World, and cannot possibly do any unrighteous thing, Iob 34. 13. Rom. 3. 6. who exactly knows all their Hearts, and Works, and cannot be deceived, nor Contradicted.

Marginal references: Psal. 97. 6.