One thing that perpetually amazes me is the absolute conviction the faithful have in their beliefs; their very specific set of beliefs are right and true and everyone else is misled, confused, wrong, and/or ignorant. The average Christian is abundantly certain that Jesus is the Son of God, the messiah foretold in the old testament, that he performed miracles, that he died on the cross and rose from the dead to save us all from sin, and that the Jews, the Muslims, the atheists/agnostics, and everyone else is wrong. Almost every religion or belief system takes this same hard line approach. And what I can't get past is the tremendous chutzpah required to completely ignore the fact that every other religion that is now or has come before felt exactly the same way about their God(s) and their rightness. How arrogant it is to say, "My experience is real, but everyone else's experience is wrong, and maybe based on nothing." Jewish people absolutely believe their experience of God is real. The Muslims absolutely believe their experience of God is real. The ancient Greeks absolutely believed their experience of Gods was real. The ancient druids absolutely believed their experience of God was real. But apparently only the [INSERT NAME OF YOUR RELIGION HERE] and its narrow, modern day interpretation of itself is right, everyone else is wrong. That arrogance strains credulity, it suggests to me that strong belief must necessarily be suspect because it cannot allow or acknowledge our rich human history of flawed thinking and errant conclusions. Religions cannot all be right, and in some sense that means the competing ones remove themselves from my consideration by canceling each other out.
There are of course some more syncretic, universal belief systems that do allow all religions to be varying shades of simultaneously true, in as much as they all are simultaneously valid personal interpretations of some universal divine; any errors or contradictions between them are creative license by the cultures who created them. This posted reason for disbelief would not apply to these particular faiths, though others reasons would.