Luke Warm Christians?

Is there such a thing as a luke warm Christian? I've heard a couple of handfuls of sermons about luke warm Christianity during my life. Those sermons usually disturbed me because most of the time the speakers talked about how not to be a "luke warm Christian". The problem I had is that there is no such thing in the Bible. You're either a Christian or you're not. There is no in between. Why deceive people into believing they can be something that doesn't exist?