I disagree with most of the Christian positions on social issues. I have serious moral quibbles with much of Christian doctrine. Significant parts of the Bible are reprehensible; the Passover story is particularly appalling. But, those are beliefs that developed after I stepped away from Christianity and examined it objectively. They're not the reason I stopped believing.
The first crack in my faith came while in jail. I was visited by members of a local Charismatic church. I had always believed that speaking in tongues was nonsense, but they convinced me to do it anyway. They were convinced that I had had a spiritual moment; I knew I was just spouting gibberish to make them happy. This presented a serious problem for me. I had always just accepted that what I was told in church was true. But, here were people just as sincerely telling me something different. If they were wrong, how could I be sure that what I had been taught was right.
In the hospital, few if any patients were actively religious. I attended the weekly chapel services, but only because I was the only pianist who could accurately sight-read whatever hymns the chaplain happened to choose that week. The chaplains were a varied bunch from assorted Protestant traditions. Without active support of my particular belief system, I had ample time to consider just how much of what I believed was simply based on personal choice.
Then came college. World religion class required me to read the Koran and other assorted holy books. I began to realise that Christianity was just one of many religions; all of which had followers who were just as certain that their religion was right as I had been about mine. A class on the Prophets took me outside the safe for Sunday School parts of the Bible. For the first time I realised that I was appalled at what I was reading in the Bible.
The final blow that shattered my faith was just after I graduated college. I was attending church with friends. The church youth group was detailing their experiences speaking in tongues during summer camp; I realised that I found it total BS. That was the last time I tried to pretend that I could still be a Christian. As much as I wanted the fellowship, I couldn't lie to myself and pretend that I believed something I didn't.
It was still several more years before I could fully accept my lack of belief; childhood conditioning is hard to shake. I wasn't even one hundred percent certain I wanted to shake it; there are a lot of advantages to being a Christian. Now, years later, my doubts have been overcome; I can confidently assert that no version of the Abrahamic God actually exists. But, it was by no means an easy journey.