by being reincarnated or summoned there is a pretty good or very certain chance that everything they've believed in up until this point is wrong?
Tbh, any religious person can have such a moment in real, daily life. Just the fact that multiple religions exist is more than enough evidence that all of them are wrong. Why should the one true god tolerate the existence of religions that worship a false god?
The people who can believe in god are already blind to being convinced otherwise. If they woke up in a world where their religion didn't exist, they would deny that reality and think something like, "Oh, God must have put me here to spread the word." or "This world is more than I expected to exist, but it is still within God's power to have created it. This changes nothing." That is 'Faith'.
I also mentioned the absence of faith I think would be really world-shattering for an atheist to meet an actual godlike being. That straight grabbed their soul and brought it to a different plane of reality.
Meeting a Godlike being is NOT the same as meeting God. An atheist (as opposed to an agnostic who holds reasonable doubt) actively denies the existence of God. Meeting a Godlike being can be chalked up to 'sufficiently advanced alien life form' or something similar. It's certainly powerful, but it's not really the big G.
I didn't quote whoever brought it up, but as for the nihilism that comes from knowing there's no punishment... Actually, I'm not sure people can change so easily. If you grew up looking over your shoulder for Santa Claus, even if you suddenly realized he doesn't exist, you'd keep looking over your shoulder.
There might be some people who genuinely fear God's Wrath while also being hopelessly immoral. Somehow I doubt it's a large percentage of people though. Most either already don't believe and will stick to their current behavior or genuinely want to follow the rules. Being told there's no rules will cause them to make up new rules to follow.
If I had anything else to say, I got bored and forgot what it was.