Why do Christians think that they are the only right ones?

So I ask this question to whomever I have a debate over religion with. When it comes to Christians, they tell me, that their way is the only right way. It doesn't make sense to me, when the only defense they have for their claim is, "because the bible says so." What I don't quite understand is, why do Christians think it is okay, to literally desecrate the thousands of years of history on the eastern side of world, as if it never existed. My girlfriend is a Christian, and that is the only thing that is going to keep me from taking it a step further. Mainly because, Christians do not want to understand the rest of the world, their festering ignorance is going to lead to the demise of their own self respect. So someone answer me! WHY are Christians LIKE THIS!