Albert Barnes Commentary Psalms 50:6

"And the heavens shall declare his righteousness; For God is judge himself. Selah" — Psalms 50:6 (ASV)

And the heavens shall declare his righteousness – They will make it known, or announce it. That is, the heavens—the heavenly inhabitants—will bear witness to the justness of the sentence, or will approve the sentence. See the notes at Psalm 50:4. Compare to Psalm 97:6.

For God is judge himself – The judgment is not committed to mortal men, or even to angels. Creatures, even the most exalted and pure, might err in such a work as judging the world. That judgment, to be correct, must be founded on a perfect knowledge of the heart, and on a clear and complete understanding of all the thoughts, motives, words, and deeds of all people.

It cannot be supposed that any created being, however exalted, could possess all this knowledge, and it cannot be supposed that any created being, however pure, could be so endowed as to be secure against error in pronouncing a judgment on the countless millions of people. But God combines all these in himself: a perfect knowledge of all that has ever occurred on earth, and of the motives and feelings of every creature—and, at the same time, absolute purity and impartiality. Therefore, his judgment must be such that the universe will see that it is just.

It may be added here that, as the New Testament has stated (see the notes at Psalm 50:3), the judgment of the world in the last day will be committed to the Lord Jesus Christ, and the considerations just suggested prove that he is Divine.

The immediate point in the passage before us is that the fact that God will preside in the judgment demonstrates that the acts of judgment will be right, and will be such as the heavens—the universe—will approve; such that all worlds will proclaim them to be right. There is no higher evidence that a thing is right, and that it ought to be done, than the fact that God has done it. Compare to Genesis 18:25 and Psalm 39:9.