If thousands of years ago the Old or New Testament had descended slowly from the heavens as a blinding light, settling on the ground and revealing itself to be universally intelligible words (readable by all men of every language) printed on indestructible paper as yet unexplained and unreproduced by scientists, then I would be strongly inclined to believe its contents are the unerring truths and wisdom of God. That's not what happened, however. Instead the Old Testament was thought to have been composed over some thousand years, from the 12th century BC to the 2nd century BC, passed down for much of that time by word of mouth. Not surprisingly subtly and significantly different versions of many of the stories/books exist, and various churches have different opinions about which books are valid and can be included. Thus it's very hard for me to imagine the Old Testament is the inerrant and factual testament of God. The New Testament has a similarly complicated history, not having been begun until well after Jesus' death, and then composed of varying, evolving books coming into and out of favor for some 1500 years or so, until stabilizing somewhat in the 16th century. Again, hard for me to see an evolving text subject to the apparent whims of various ages being the inerrant and factual testament of God. I cannot understand, knowing what biblical scholars do about the creation and evolution of the bible, how anyone can believe the bible is literally true.
One of the curious things about many religions is their ability to interpret events to fit a positive outcome. No matter what the event.
A woman prays to God asking that her father be spared from the infection ravaging his body. If the father gets better, God is credited. If he dies, it's okay, because his dying was part of God's unknowable divine plan. Not every religious tradition couches things in exactly these terms, but they all have in them the very same conveniently indisputable doctrine of optional but universally positive involvement. People are encouraged to ask God for things, but He is never expected to act. How very convenient that is to preventing one from easily disproving His existence. We could similarly invent and then be unable to disprove the existence of anything else we might wish; and I can't help but find that style of existence deeply suspect.
Two related accounts:
My father went on a road trip recently, and very near the end of his 3,000 mile journey, about 50 miles from home he was pulling up to a light, and one of his front wheels fell off. His reaction, as he related it to me, "We passed through canyons, along cliffs, and at 75 MPH on highways and we would have died if the wheel had fallen off then! God was protecting us!" But the many other people who recently died in car accidents along the same route he took, God apparently chose not to protect them. Such subjective selection in which acts are credited to God I find frustrating.
I remember a while back watching a story about a family escaping unhurt from a terrorist bombing that killed dozens of others, and their statement was something like, "We just knew that God was protecting us, that we'd make it out, that he still had plans for us." The other people had apparently outlived their planned usefulness on this Earth (and were needed in Heaven). I am troubled by this woman speaking so confidently about how her family was so deserving of God's protection while seemingly not recognizing that others were equally deserving, just not as fortunate.
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.
The faithful like to argue that the universe could not have created itself out of nothing in the moment of the Big Bang, that the idea is absolutely nonsensical. I agree. But what seems so curiously hypocritical to me is that those faithful are promoting exactly the same sort of nonsense, that either God somehow created Himself out of nothingness or that He somehow always existed. Both those something-in-spite-of-nothing beliefs make no more sense than the prevailing scientific theories about how matter and energy might have come to be in the Big Bang. Forced to choose between multiple ridiculous ideas, I'll choose to believe the most direct and least complicated one.