I went through a three year journey from Hell. It actually started by reading the entire Bible. I wrote down difficult passages and took them to my pastor and / or sunday school teacher. The list of questions got longer and the answers got vaguer. My process was deeply involved Christian to doubter (deep guilt for doubting), to deist to finally Agnostic Atheist.
It took me about a month after leaving my church, to admit that I was an atheist.