Christianity is, by technical definition, a theory.
No, it most certainly is not. Not only are they not in the same ballpark, they aren't even the same sport. Theories are not assertions of fact, nor are they blind guesswork. Religion, on the other hand, is exactly that.
A theory that holds no less, and no more water than your precious Big Bang theory.
You really don't get it, do you? The Big Bang theory isn't "precious", nor is it sacred, infallible, or unchanging. It is simply the theory that best fits the availible evidence.
That’s why we call it faith.
There is no place for "faith" in scientific theory. Theories are, by their very definition, supported by many lines of evidence and observation... That's not faith. The Big Bang theory is falsifiable, as are all scientific theories, if christianity were, as you so ignorantly claim, "just another theory" then what makes or would make christianity falsifiable?