Faith is a fervent desire to believe that a collection of pleasant beliefs are ontologically correct and true. Doubt is the nagging feeling that the beliefs of faith are ontologically innaccurate and false. Faith will always lure the gullible because the emotional rewards of certain beliefs is quite strong (like God loves you). Doubt will always plague the faithful because experiential evidence often directly contradicts the beliefs of faith (like the question of evil).

When I was a young child, I used to believe in Santa Claus. It was a pleasant belief that said I was evaluated positively by an objective, outside observer who rewarded me for my good behavior. Santa Claus loved me and gifted me appropriately. Unfortunately, one Christmas morning I woke up at 4:30, and I caught my mother putting gifts under the tree. Suddenly, I had to doubt my beliefs about Santa Claus. I was dejected.

The Christian God is Santa Claus, except that Christians spend their whole lives trying to deny that he's just a made up person.