How will Shamayim tell His virtue?
Radak: (a) Shamayim and its legions stand forever, unlike legions of the land - a generation comes and passes. One who knows the time in which he is, does not know the past. Shamayim and its legions that were in the past, they will be in the future on the day of judgment and gathering the exiles. They will tell the Tzedek that Hashem did with Yisrael - He did not finish them off in Galus and annul His Bris with them; He took them from Galus. (b) He waged Yisrael's case against the nations. "Va'Yagidu" is past tense in place of future.
Malbim: In the land, people err and think that Hashem desires thousands of rams. In Shamayim, that there is no eating or drinking, they will tell Hashem's virtue in this law.
Why does it say "Selah"?
Malbim: This completes the matter of calling [to Shamayim va'Aretz]; now begins the rebuke.
Emes Keneh: The Gematriya of the initial letters of "Shofet Hu Selah" is 365. This hints that He judges man every day of the year (Rosh Hashanah 16a).
