Initially almost no evidence will be required for at least one individual to believe in something ... until almost all believe. Mass belief in something is akin to mob mentality, once you get to critical mass people don't believe because there is good evidence to do so, they believe because everyone else does ... then for at least one individual no amount of evidence could make them believe.
This individual, like the first believer, is the seed of the counter belief ... not in the sense that they are a messiah/guru figure ... such people are almost always johnny come lately's cashing in on what is already there rather than being true visionaries .. their only vision tends to be what they can make out of it. Jesus and Buddha being prime examples.
I suspect that there is a evolutionary mechanism for this that ensures that we don't all go running off the cliffs to our deaths however plausible the reasons to do so might be.
What I guess I'm saying is that the vast majority of people require no evidence at all to believe in the majority of things they believe in. More so I'd claim that most rationalisations for said beliefs are almost always post hoc justifications.