The film's success lies in the fact that the conceptual idea (contained within the title) has been thoroughly thought through and persuasively argued.
In order for the Church to defeat the action it must admit that God doesn't exist; a theosophical Catch-22.
Abrasive, funny and thoroughly good-hearted (even if the "miraculous" denouement trades a little too much on the audience's goodwill) it is the treat of the week.