1 Innkeeper: I'm dead? You're here to collect my soul?
1 Death of Insanely Overpowered Fireballs: YES.
2 Innkeeper: But I'm a worshipper of Photus, God of Light. I should be going to the Shiny Happy People Place!
3 Death of Insanely Overpowered Fireballs: PHOTUS DOESN'T EXIST.
3 Innkeeper: But he's got clerics! They heal wounds!
4 Death of Insanely Overpowered Fireballs: OH, SO PRIESTS TELL THE TRUTH ABOUT THIS SORT OF STUFF NOW, DO THEY?

Dungeons & Dragons campaign idea:

It starts like a regular D&D campaign, with the players assuming all the usual stuff about how magic works and the existence of pantheons of various gods who grant powers to clerics. The cleric in the party however has a backstory that he is on some sort of acolyte probationary period as demanded by his god, and that he will only gain spells at second level. (This is how the old Basic Dungeons & Dragons edition of the game worked by default, so it's not too much of a stretch.)

The heroes have some adventures and gain experience, and after a while they qualify to advance to the second level of their character classes. The DM takes the cleric character's player aside and tells him that due to some new regulation or whatever in his church, the granting of spells will be delayed another level. But the church, being dedicated to the god of war, offers combat training in stead of spellcasting. Or being the god of trickery, offers pickpocketing and lockpicking training. Or being the god of love and beauty grants a charisma bonus. Or whatever - something that balances the lack of spellcasting for another level.

Continue adventuring. At third level, the cleric's church offers some other excuse. No spells, but some other sort of class advancement option. By now the PCs might be getting suspicious. If they investigate or otherwise press the issue, they eventually learn that nobody in the church has the ability to cast spells. It's all an elaborate and long-running ruse by church elders to convince the public that they have magical powers, using a combination of hearsay, trickery, and using actual wizards to simulate divine magic.

The PCs slowly discover that all of the churches in their world are running the same deception. There is in fact no such thing as divine spellcasting or divinely granted powers. They live in a world with no active gods at all.

What they do with this information could drive the campaign for some time. Were there active gods who disappeared or stopped granting powers for some reason, and the churches built a wall of deception to maintain the status quo? If so, can the gods be contacted and returned somehow? Or have there never been gods, and the priests simply constructed religions wholesale as a means to gain power? And in this case, what should be done about it?