1. Authority -- NOT -- it was compiled, copied, edited, codified and translated by men. Men with agendas. Over the hundreds of years it was put together there were perhaps hundreds of "hands" tinkering with the unalterable "holy" words.
2. Inconsistency. Two Adam & Eve stories. Two genealogies for Jesus. Discrepancies amongst the Gospels. Too many inconsistencies to mention, and anyway The Skeptics Annotated Bible did it already.
3. God's nature is fickle and inconsistent. He is forgiving or resentful depending on the situation. Sometimes he tinkers in the Affairs of Man and sometimes not. He wants you to follow his rules, but then there's the parable of the prodigal son. He made the world and all the animals, including people, and yet made all sorts of really horrible and stupid things. For instance, why do humans have "tail" bones if we don't have tails? Having broken mine I can tell you I'd rather not have it. If he wanted us to protect the useful parts of our spine in a fall, then why put nerve endings there?
4. Miracles. They have no corroboration outside of the Bible. They could have been faked or made up as propaganda or exaggerated over time. If Jesus really did walk on water, how do we know he didn't go there in advance and put a table just under the water line? How do we know there wasn't a sandbar there? And yet he couldn't make a fig tree yield fruit out of season, which would have been a more difficult feat than appearing to be walking on water. Couldn't pop the nails out of his hands and feet and jump off the cross, either.
5. Revelation. Dreams, voices, visions... they are all reminiscent of what today would be considered symptoms of psychosis. If they're psychotic symptoms now, they very likely would have been then, if they even happened. Primitive people can't be faulted for believing that dreams or migraine auras or psychotic breaks came from some supernatural entity, but we shouldn't believe them now. The opposite is possession by an evil spirit. Also mental illness that was misunderstood by bronze age superstitious people.
6. Scientific inaccuracy. God could have revealed the truth about the Sun revolving around the Earth, at the very least. All of God's words seem to be consistent with what humans would have known at the time, and not at all revelatory or helpful. Every human culture has a creation story. The Judeo-Christian-Muslim one is just one of many with no claim to accuracy in the least.
7. Similarity to mythologies in other Middle Eastern religions. Just a little too many similarities to dismiss. Mithras, for example.
8. Speaking of Paul, Paul's role is a little too important in early Christianity. He never met Jesus, yet he supposedly explains Christianity with authority. He has a completely different message from Jesus' supposed words. A lot of Biblical inconsistency right there. Why should anyone believe anything he said? None of it was of a nature that couldn't have come from psychosis, imagination, or calculation. If he was divinely inspired, he could have set people straight about the Sun, for instance.
9. The Book of John. Written much later than the other "gospels" and seems very biased. Coincidentally, "fundamentalist" Christians are fond of quoting John. They like his brand of Christianity so much that their whole theology would crumble if that "book" was taken out of the Bible.
10. Disturbing "morality." Over and over there are truly disgusting examples of God or his favorite people doing the most heinous things. The worst of all for me is the central tenet of Christianity: that Christ was sacrificed for the sins of mankind... all of us or some of us, depending on your denomination. This means that a "loving" God practiced scapegoating, punishing his one good child for the sinfulness of all the others. No actual sinning is required to be defined as a bad child, since sinfulness is inherited. Inheriting the "sins of the fathers" is also immoral. Other repugnant practices are portrayed without any negative judgment: war, genocide, polygamy, rape (but only of women!), and slavery to name a few. Then this "loving" God will send everyone who doesn't say they "accept" him to eternal fire and pain. What kind of "love" is that?
10a. Cannibalism. Yech! You can say it's just metaphorical and wine doesn't really turn into blood, but still, it's a repulsive practice and extremely barbaric. Early Christians already had the practice of baptism for the cleansing of sins, so they really didn't have to have eat their god in a repulsive ritual meal. That practice is also waaaay too similar to that of other religions to be taken seriously as a true historical tale.
I could probably come up with more but these are the big ones for me. Much ink has been spilt explaining the problems in the Bible. People get Ph.D.s in something aptly called "apologetics." They call the Book "god-breathed" or inspired rather than taking it as the literal gods-ear-to-man's-pen truth, because they know deep down it's really a bunch of ridiculous nonsense. To believe in this book is to believe in a God that's mercurial, vengeful, narcissistic, and possibly insane.
Or... you could believe that the Bible is just like all the other holy books of all the other religions, just a bunch of fairy tales with supernatural buddies and/or bullies as the main characters.
Some of my smaller grievances don't get much attention, but for what they're worth:
- If all of creation was 'good' then wouldn't Adam & Eve have been exiled to a pretty nice place?
- Why is it an "abomination" for men to have sex with men but not for women to have sex with women? Isn't that also homosexuality?
- Why was there no judgment against Lot's daughters after they got him drunk then got pregnant by him? His wife was turned into a pillar of salt just for looking over her shoulder at her former home. That seems a little harsh.
- If Jesus' conception was immaculate, then why does he have a genealogy traced through Joseph's side of the family?
- And the fig tree, wtf? Why doesn't Jesus regret his temper tantrum if he's such a great guy? Come to think of it, why did he smite the tree in the first place? Is this some kind of metaphor that a woman who won't have sex during her off-cycle will be smote?