On Religious Lies

Today I came across a most persuasive argument for religion; and it came from a work of fiction by Kurt Vonnegut, in his book Cat’s Cradle:

Anyone unable to understand how a useful religion can be founded on lies will not understand this book either.

So be it.

Cat’s Cradle is based on a made-up religion.

Why does religion have to be based on actual historical truths to be accepted for what it is? Does God really need to exist for us to believe in His existence?

And, ultimately, what is wrong with believing a lie?