The worst thing that ever happened to me while being religious didn't even actually happen to me directly - it happened to my mother. She was abused by her alcoholic Southern Baptist lay preacher father.
This experience affected her and, through her, my father, my siblings, and me. I won't go into all the gory details but suffice it to say that even though I never met my grandfather his actions have profoundly impacted my life. I was taught that there is no god and believers are not to be trusted.
As a teenager and young adult I dutifully rejected my parents' ideals, including atheism. I sought belief in some kind of higher power to avoid my parents' fate. My dad did the best he could, but my mother is not a happy person. I thought her unhappiness came from being an atheist, especially after I encountered believers who told me that non-believers can't know joy.
In the end I realized for myself that there is most likely no god and that I am responsible for my own happiness no matter what has happened or what will happen. I realized my mother's unhappiness does not stem from being an atheist, but from a multitude of factors including religion-shrouded abuse.
I have told this to several Christians who have boldly asked why I do not believe. Their responses range from, "I am sorry that happened to your mother, but that person was obviously not a true Christian," to "That is awful, but Jesus still loves you and wants you to believe in him." They seem incapable of just accepting that I am not ever going to believe. I already understand that my grandfather did not represent Christianity, but he did to my mother. That's all that mattered.
They don't get it - it's too late. It is not because of what I was taught as a kid, but what I found out as an adult. If I wanted to play make-believe, I could pretend that I believe. I could tell myself that I believed until I did believe. I prefer to live in the real world which, in absolute defiance of the message my mother got and kept about life, I know to be a place capable of amazing beauty, compassion and love without the necessity for a supreme being.