Ricky Gervais' The Invention of Lying begins as a single-premise comedy: wouldn't it be funny if everyone was brutally honest, all the time? (Answer: sorta) But it becomes The Invention of Religion about halfway through. By boldly asserting that Religion doesn't exist without lies, Gervais isn't being particularly subtle about his thesis.
There is one point that I think bears more examination: when the film's characters are exposed to religion for the first time ever, they take it much differently than someone in our world; they take it deadly serious. The reasoning is obvious; if you have a chance to enjoy eternal happiness in the future, all other parts of life become trivial in comparison. So when the film's characters are told they are allowed two sins (Like baseball, three strikes and you're out) some characters wisely decide to drink themselves into a stupor, since it helps keep you out of trouble sin-wise, and it accelerates their deaths, after which, their "life" will be better.
Why don't we see people "gaming" the system in our world? Why don't people spend their formative years figuring out how to get into heaven, then spending every bit of labour for the remainder of their lives trying to secure that place in heaven?
They Don't Believe.