I loved Nerdy's rant about Mollymauk calling the Traveler imaginary. I've played with a few players that have tried the doubt the existence of the party Cleric's deity thing, but most point out that D&D Clerics have actual evidence their god is real.
My favorite D&D webcomic even did an interesting flip this idea on it's head right after the party leader came back from the dead. He and his girlfriend have this great philosophical discussion about what it'd be to live in a universe WITHOUT empirical proof of gods and an afterlife.
https://www.giantitp.com/comics/oots0669.html