"Christianity, if false, is of no importance, and if true, of infinite importance. The only thing it cannot be is moderately important."

[JWeinCom]

People are really taking this out of context...

What Lewis is saying is clearly not that Christianity as a concept and historical entity is unimportant if not true. He's saying that if you believe in the core tenet that there is a heaven and hell where you will spend eternity, and which one you go to depends on whether or not you're doing what Jesus wants, then it is bizarre for anyone not to go balls deep with it. If you really truly believe this is where you will spend countless billions of years, anything less than full devotion is insane.