John Calvin 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. The Jews were arrogant enough to imagine that their empty and fanciful service was the perfection of righteousness; but they are here warned by the prophet that God, who had seemed to overlook their folly, was about to reveal his own righteousness from heaven and expose their pathetic schemes.

"Do you think," as if he had said, "that God can take delight in the mockery of your deluded services? Though you send up their smoke to heaven, God will make known his righteousness in due time from above and vindicate it from the dishonor inflicted upon it by your wicked fabrications. The heavens themselves will attest to your treachery in despising true holiness and corrupting the pure worship of God. He will no longer tolerate your baseless slanders against his character, as if he did not notice the hostility hidden beneath your pretended friendship."

There is thus a forcefulness in the prophet’s manner of addressing his subject. People are inclined to admit that God is judge but, at the same time, to fabricate excuses for evading his judgment. It was therefore necessary that the sentence God was about to pronounce should be defended against the pointless objections that might be brought against it.