If there are multiple creatures with the same power on the battlefield, you may choose any of them, even if one of them wouldn't be destroyed. For example, if you control a 2-power creature with indestructible and a 2-power creature with no abilities, you may choose the second one even though the first one can't be destroyed by Celestial Judgment.

All creatures that aren't chosen are destroyed simultaneously. They see each other leave the battlefield for the purpose of triggered abilities.

No creatures are targeted by Celestial Judgment, so all choices are made as the spell resolves. Notably, no players may respond or take any actions between the creatures being chosen and the other creatures being destroyed.