Once a child has been indoctrinated into a religion, their eventual deconversion is usually the result of some combination of:
#1 studying the whole bible, including the horrible things like the slaughter of infants on gods order (joshua 6), genocides, infanticides, etc. There are so many disgusting justifications that it requires supernatural credulity to swallow it. Only a desperate or indoctrinated mind will do so.
For some perspective on this (assuming you are Christian) think about how you feel towards the claims about the Qur'an (or writings about Zeus, etc). I feel *exactly* the way about your claims about your bible.
#2 an intensive study of modern sciences (history, physics, cosmology, biology, chemistry, etc) and a comparison of the depth and details of scientific knowledge verses the extremely sketchy and often erroneous accounts in the bible. It is clear that the authors of the Bible knew absolutely nothing more than would have been expected from their respective time periods. Do NOT fall into the trap of thinking ancient people stupid, they lacked certain tools and knowledge that we build upon today but many of them were extremely intelligent even if they were limited by their cultures. Also, don't fall into the trap of believing "All Ancient People Thought X" because that is just as false about ancient people as it is about people today. Not all ancient people believed the earth was flat, many understood very well that it was round and had even given a very good estimate for the circumference (http://en.wikipedia.org/wiki/Eratosthenes circa 240 BCE) and they had made extremely detailed observations of the sky for centuries.
#3 a comparative study of religions including the some 3000 different sects of "christian" beliefs (if even the most devout "Christians" cannot agree on key doctrines like transubstantiation, then what validity is there in the bible?)
#4 the flat out errors, insane claims, and conflicting stories that require extensive apologist justifications to prevent the more critical from outright revolt.
#5 studying the history of Christianity and how utterly evil and corrupted the entire church was for a 1500+ years of being in power. Yuck
#6 complete lack of contemporaneous, eye-witness evidence for Jesus. Not even the "gospels" are eye-witness accounts, nor were they written in the time of Jesus but some 30 years later. Nor are any of the outrageous claims of events documented by any historians (e.g., the claimed multitude of Saints rising from the dead).
#7 the complete failure of prayer, 1 Kings 18, and other specific claims of the bible
From a purely intellectual/logical view I cannot understand how sane, rational, and reasonably well-educated persons can look at the facts and not reject the claims of Christianity (Although I do understand it from an evolutionary, sociological, cultural and psychological standpoint).
I can kind of understand the feeling that there is some kind of "god" in a deistic or pantheistic sense, but such claims are ultimately not justifiable because they cannot be distinguished from the null hypothesis.