Albert Barnes Commentary Psalms 50:4

"He calleth to the heavens above, And to the earth, that he may judge his people:" — Psalms 50:4 (ASV)

He shall call to the heavens from above - He will call on all the universe; he will summon all worlds. The meaning here is not that he will gather those who are in heaven to be judged, but that he will call on the inhabitants of all worlds to be his witnesses, to bear their attestation to the justice of his sentence. See Psalm 50:6. The phrase “from above” does not, of course, refer to the heavens as being above God, but to the heavens as they appear to human beings to be above themselves.

And to the earth - To all the inhabitants of the earth; “to the whole universe.” He makes this universal appeal with the confident assurance that his final sentence will be approved, that the universe will see and admit that it is just. See Revelation 15:3; Revelation 19:1–3. There can be no doubt that the universe, as such, will approve the ultimate sentence that will be pronounced on mankind.

That he may judge his people - This means that all these arrangements—this coming with fire and tempest, and this universal appeal—will be preparatory to judging his people, ensuring the judgment is conducted with due solemnity and propriety.

The idea is that an event so momentous should be conducted in a way suited to produce an appropriate impression. It should be conducted so that there would be a universal conviction of the justice and impartiality of the sentence.

The reference here is particularly to his professed “people”—that is, to determine whether they were truly his. This is the main subject of the psalm, though the “language” is derived from the solemnities appropriate to the universal judgment.