That sounds familiar to me. I used to be on a mission from god. Satan used to come into my bedroom at night and discuss my fate with two of his 'angels'. I saw a holy war between demons and angels, and demons came around me constantly. Voices told me the "cosmic truth". The world was slated for flood, or somesuch thing, and I was chosen to survive a test that would destroy most of mankind. I dreamed of armageddon, and the part I would play in the end of this world, and the beginning of the new one. And the meek would inherit the earth. I had become hyper-religious, as well as having a lot of seemingly way-out beliefs that scared others, but I thought were my salvation. At one point I belived I had "fallen", that only evil spirits wanted me because I was so evil inside that nothing good would come near me. Homicidal and suicidal thoughts happened frequently at this time.
Then I learned more about mental illness, and realized these thoughts, hallucinations, and conterfactual beliefs were derived from chemical imbalance, which is worsened by the hormone changes at puberty. Which also accounted for the paranoia, depression, psychotic symptoms and mood swings. It took me a long time to figure that out, and in the mean time I was hospitalized and medicated. It took me a longer time to realize that I was ultimately responsible for my own mental health, and that the more I bought into these ideas, the less chance I had of getting better.
I hope that you can get help and come to the other side of this. Take a look at the article Malady posted, the chemical imbalance one. It may explain some of this for you.