I was brought up as a Mormon, and I was very serious about religion. I didn't just go along with it because my family was Mormon. I really believed God existed.
Now, sinning is a barrier to the best eternal life available in the Mormon religion, and I wanted the best. Of course, everyone sins, but we could ask for forgiveness, and I would do so routinely.
However, there was one sin I read about for which there was no forgiveness, which is denying the Holy Ghost. I was very afraid of committing this sin. It seemed like I might accidentally do it in my mind. I might think the phrase: "I deny the Holy Ghost."
Now people would try to say "Oh, no, you really need to know that God exists before you can deny him in an unforgivable way," but I felt like I knew God existed, sortof. In any case, this did not seem like something to risk leaving to human interpretation of scripture.
The fear grew and for perhaps a week during Junior High School, my mind was almost entirely consumed with trying to make sure I didn't deny the Holy Ghost.
Eventually a fuse broke in my mind, and I figured the fear was worse than hell, so I allowed myself to think the phrase: "I deny the Holy Ghost."
At this point, I was still very much a believer, but this is one of a number of events that led me away from the Mormon faith.
Recently, I met someone who also went through a similar ordeal. They too were very religious growing up, and had now completely fallen away. The idea came up that making denial of the Holy Ghost into an unforgivable sin is a very powerful protection mechanism for the religious meme -- if you want to think about religion as a meme. It essentially says, the more you believe the religion is true, the worse it will be for you if you leave it. However, in both of our cases, it had backfired, causing a fear so intense that it overshadowed the benefits offered by the religion.