If you believe in God and an eternal afterlife, and you have nuclear weapons, it may seem acceptable to "virtuously" engage in nuclear war against an evil opponent, even risking extinction of humankind.

Revelations is allegorical, after all--maybe it's an allegory to nuclear war.

So now the ledger looks like this: if you accept God and you're right, it's a huge win...but if you're wrong, you risk the biggest possible loss. Toss in a little temporal discounting to keep things finite (or allow human life to potentially continue forever, ignoring our best but admittedly not-that-deeply-verified ideas about the ultimate fate of the universe), and you actually need to start paying attention to the probability of the hypothesis being true.

Of course, there was always a tradeoff--accepting God is not a purely vacuous step, practically. But it is only since the advent of nuclear weapons (and similarly apocalyptic technologies) that it starts being really really clear that a false belief could have a truly immense negative consequence.