It is said that the purpose of the Christian sermon is to convince the Christian that he does in fact believe in Christianity. That is, to give the Christian faith that he has faith; to remind the Christian what he believes in. And why does the Christian need to be reminded? - because it is so easy to forget you believe something, especially when what you believe in isn't true.
Personally, I think that to ask someone to believe in the doctrines of Christianity is to ask too much. Thus I hold that the sermon is designed to help the Christian forget his belief that he doesn't really believe in Christianity . . .