I remember thinking these 'existential' thoughts that cause depression my first day of pre-school. I am not as intelligent as a lot of people, but I know that I'm not unintelligent. I began failing my classes in school when I was 9, and my depression was beginning to really develop. I would walk around the playground, by myself, thinking. By 15, I had renounced my belief in God and refused to be brainwashed by anyone who wanted to tell me otherwise.
I'm 25 now and after a lot of drugs and alcohol, I believe in God, again. I read the Bible, not as often as I should, but at least I read it. It makes me feel better.
For those of you who are 'former Christians', I recommend you try to bring it back into your life. It does help, I promise.