I agree that literature shouldn't have a set of requirements.
I'm not a religious person, but in my mind calling the Bible 'literature' sounds like a praise. I would also consider the Koran literature, as well as any other religious text. You don't have to be a believer of a religion to think that religious texts are beautiful pieces of literature. I think literature is any sort of writing that interests people; pretty much any published work at all. A person doesn't have to like the bible or Dr. Seuss but would have to admit that they are writings of interest to others so therefore they are literature to some people. You can think the ideas in The Communist Manifesto are completely horrible and wrong, but would have to admit that it is also literature to those that believe in it and even to some that don't believe in it but believe it is an important, well written document.
It doesn't need a specific definition. It is what it is but can also be considered something different to someone else.