I'd go with "Hey, it's God. I actually AM real, but I'm heading out. You guys are on your own from now on, no more afterlife or anything. So be good to each other! Alright, I'm out. (oh, and just some advice? you might want to get ahead of this climate change thing)"
Give them all a vision of the Earth from space. Look at that, all ya sonsa bitches.
“You develop an instant global consciousness, a people orientation, an intense dissatisfaction with the state of the world, and a compulsion to do something about it. From out there on the moon, international politics look so petty. You want to grab a politician by the scruff of the neck and drag him a quarter of a million miles out and say, ‘Look at that, you son of a bitch.’”
— Edgar Mitchell, Apollo 14 astronaut, People magazine, 8 April 1974.
The experience of being the victim of real violence. It's pretty clear most people don't really get it. And it's weirdly common for people to have romantic ideas about fighting, revenge, punishment, war, and even torture. I think it would be eye opening.
Is "kill yourself immediately" too dark? Probably. But if like three billion people died suddenly and without a massive environmental impact like nukes or some other poisoning of the world, could solve a bunch of problems.
'Gender critical' theory. Everyone should have this theory, as originally defined, planted in their head. Men would only associate 'masculinity' with their role in providing semen for procreation. Women would only associate 'femininity' with their role in providing eggs for procreation.
There would be no gender-non-conformity because there would be no ideas about gender to conform to. A minority of men would, as an everyday thing, wear dresses. It would be impossible to verify that someone was a woman or not (short of x-raying their uterus).
The important thing, is that if this happened, the people currently claiming that they're 'gender critical' would lose their freaking minds.
"A spectre is haunting Europe — the spectre of communism. All the powers of old Europe have entered into a holy alliance to exorcise this spectre: Pope and Tsar, Metternich and Guizot, French Radicals and German police-spies.
Where is the party in opposition that has not been decried as communistic by its opponents in power? Where is the opposition that has not hurled back the branding reproach of communism, against the more advanced opposition parties, as well as against its reactionary adversaries?
Two things result from this fact:
I. Communism is already acknowledged by all European powers to be itself a power.
II. It is high time that Communists should openly, in the face of the whole world, publish their views, their aims, their tendencies, and meet this nursery tale of the Spectre of Communism with a manifesto of the party itself.
To this end, Communists of various nationalities have assembled in London and sketched the following manifesto, to be published in the English, French, German, Italian, Flemish and Danish languages."