There's no situation which would convince me that Christianity had a 100% probability of being true, because the idea that the entire scenario since I first encountered evidence of Christianity being true was a hallucination or that I was a Brain-in-a-Vat could never be disproved, but I can easily imagine scenarios that could make me raise my estimated probability of Christianity much higher, to 50%, 90%, perhaps higher.

If I were teleported into an alternate world where world history and the like seemed more consistent with Christianity being true, I could easily envision my probability ranking to as high as my current one for Atheism, to the point that I would act based on the assumption that it had a 100% probability.