What one thing would disprove Christianity to you?

[The realest]

That's a great one. That can never be proven. Just ask Richard Dawkins "Why there ALMOST certainly is no God."

Some atheists feel the need to append "almost" to statements of otherwise categorical rejection of God because theists seize upon categorical rejection with foaming mouths screaming "PROVE HE DOESN'T EXIST".

Adding that touch of agnosticism satisfies intellectual honesty if your standard of such is complete precision, but I tend to take a practical approach to the question. No theist can prove God exists, so I do not see why I should humor them by pretending there is any realistic chance that their wishful thinking is accurate in any regard.

When the only defense you have is to appeal to infinitesimal odds, you don't have a position worth defending.

I don't have a position worth defending? Here's a defending answer: Nothing never creates something. Just look around you, and everything thing you see was created by something. Can you disprove this? We both know the answer.