The very first thing/reason that hit me in my late teens is hell (and I found this good summary of what I think at Dwindling in Unbelief).
Hell is the core of Christianity; it is what Jesus came to save us from. We all deserve to go there, and there is only one way to escape: believe the right things (just what those things are depends on who you talk to.) And, if for whatever (and however good a) reason you should die without that belief, you will be tormented forever in Hell by the god who loves you. It is as simple, cruel and absurd as that.
Here is what Charles Darwin said about it in his autobiography:
I can hardly see how anyone ought to wish Christianity to be true; for if so, the plain language of the text seems to show that the men who do not believe, and this would include my Father, Brother, and almost all my best friends, will be everlastingly punished. And this is a damnable doctrine.
It was Hell that did me in as a Christian. I, like Darwin, couldn't believe that my family and friends and billions of other nonbelievers (and religiously incorrect believers) would be tormented forever in Hell for their honest disbelief. It amazes me that anyone could.
My dad and I had many a discussion about isolated native populations that have never heard of christianity dying (2000 years worth of these people) and whether they went to hell or not. As an adult this expanded in my head to include such incredibly, humble and "saintly" people (like Gandhi for example) who would b punished because they are not the right faith??? (I mus point out that according to my father, there is "baptism by desire" in the catholic church which does cover for example unbaptized infants that die and people that don't know of christianity for example which does deal with native people to some respect - but this issue originally started me thinking about a loving god who would punish such people and lietral/fundemental interpretationists have a completely different opinion - "everyone who doesn't follow christ will go to hell" which to me makes no sense and my main point. There are also several people including my family members who believe that there are many ways to get to heaven and that other religions are not exempt - and that multiple religions worship the same god....).
Hell is indeed a damnable doctrine. Darwin, as usual, had it exactly right (thanks to Dwindling in Unbelief for writing most of that much better than I could have).
Next are biblical reasons that came up over many years of studying more and more of the bible.
Biblical accuracy and authorship - I want to know who wrote something and if it is to be trusted as a good source.
The first and most important point is that the gospels themselves are not written by any first-hand eye-witnesses but rather second-hand (maybe but more probably third or later) accounts of stories that the writer heard from someone who claims to have seen the events described; all of which begs the question why didn’t anyone write it down? The claim that the disciples were too busy makes no sense – god can inspire people 50-100 years later to write it down but not any actual eyewitnesses?
According to the New Testament gospels, Jesus' fame spread far and wide throughout his lifetime. He was known throughout
If these things were true, it is beyond belief that the historians of the day could have failed to notice: but that is what happened. Not a single contemporary historian mentions Jesus. The historical record is devoid of references to him for decades after his supposed death. The very first extra-biblical documents that do mention him are two brief passages in the works of the historian Josephus, written around 90 CE, but the longer of the two is widely considered to be a forgery and the shorter is likely to be one as well (see part 2). The first unambiguous extra-biblical references to a historical, human Jesus do not appear until well into the second century. This extends to not seeing any real evidence in the authenticity of the new testament, the lack of any roman or jewish historians even mentioning Christ (the limited works of Josephus which weren't written until 90CE are questionable/probable forgeries as well as very limited in what he actually says, and nothing was said by noteable writers such as Philo of Alexandria, Justus of Tiberius, Seneca the Younger etc who lived and wrote about lots during that time YET don't mention Christ at all), the fact that the first unambiguous reference to Christ is in the writings of Justin Martyr and Irenaeus of Lyons, around 150 CE: all of which is very difficult for me to reconcile and accept as true history.
Not one single person who actually saw Christ do any of the things wrote one word about it? Not one contemporary historian wrote about the earthquake or the eclipse that was supposed to have happened during the crucifixion either???
The gospels cannot help in proving the historicity of Jesus, since the accuracy of the gospels is itself what is in question so everyone who just accepts the gospels as fact haven’t looked at the history of the bible. Not only does the lack of ANY corroborating evidence for the miraculous events of the gospel indicate the gospels should be questioned at least but the internal contradictions suggest that their authors were not recording historical events they remembered, but rather telling a story, changing events where they felt it necessary to make a point (and how can literalists continue to think that every word is absolutely correct???).
Where was Christ’s first post-crucifixion appearance in In Galilee: Matthew 28:5-7, Mark 16:5-7 or in In
Who did Christ first appear to?
the two Marys
Matthew 28:1, 9
In the end of the sabbath, as it began to dawn toward the first day of the week, came Mary Magdalene and the other Mary to see the sepulchre.... And as they went to tell his disciples, behold, Jesus met them, saying, All hail. And they came and held him by the feet, and worshipped him.
Now when Jesus was risen early the first day of the week, he appeared first to Mary Magdalene, out of whom he had cast seven devils.
But Mary stood without at the sepulchre weeping ... and saw Jesus standing, and knew not that it was Jesus.
Cleopas and another
And, behold, two of them went that same day to a village called Emmaus.... And the one of them, whose name was Cleopas, answering said unto him, Art thou only a stranger in
And that he was buried, and that he rose again the third day according to the scriptures: And that he was seen of Cephas, then of the twelve.
Are divorce and remarriage ever allowed?
No, never: Mark 10:11, Luke 16:18
Only when the wife commits adultery: Matthew 5:32
Only when the wife commits adultery: Matthew 19:9
Only when the unbelieving partner wants it: 1 Corinthians 7:13-15
Next is the issue of morality and the bible as a guide.
The issue of morality, and those that believe morality comes only from god have to really pick and choose (and/or come up with some extremely crazy scenarios) what part of the bible to look to (and you can't use the whole "the god of the old testament was different etc.." because what would a few thousand years be to the creator of time itself - he shouldn't be so capricious and should have known that exhibiting every human emotion such as jealousy, possessiveness, revenge, hatred, warmongering etc would cause some serious doubt on his existence.
here is just one brief example (from John Loftus) of the morality issue (especially for biblical literalists):
The Bible prescribes a host of detestable 'moral' guidelines. For example, if an Israelite man desires a female captive from war, he is permitted to force her to be his wife (Deuteronomy 21:10-14). If a virgin who is pledged to be married is raped but fails to cry out, she is to be stoned along with her rapist (Deuteronomy 22:23-24), while if a virgin who is not pledged to be married is raped and does not cry out, she must marry her attacker (Deuteronomy 22:28-29). Psalm 137:9 touts the pleasure of dashing children against rocks, and full-scale genocide is proscribed throughout the Old Testament (e.g., Deuteronomy 7:1-2, 20:16).
The judeo-christian god is clearly a hateful, racist, and sexist divinity. Though Christians rightly criticize militant Islamists for aiming to kill innocent bystanders, the only difference between these extremists and the biblical god is the desired target of murder. As Sam Harris notes, "it is only by ignoring such barbarisms that the Good Book can be reconciled with life in the modern world. (for a much more detailed discussion see John Loftus's work her)