Many people grow up being told they are christians but realize the stupidity of the idea of virgin birth and such. So they cherry pick what they want to believe and what they don't want to believe and call that christianity. They see nothing odd about that, but it just makes no sense. To an outsider (atheist) that sees so many diverse claims to what defines a "christian", the only thing that gives some objective meaning to the term is the bible. To the degree a person believes in the bible, they are christians. Same with the holy books of other religions and their followers. Thus fundamentalists are most easily identified with their religion in spite of the fact that plenty of folks call themselves christians that may not even believe Christ existed. And many of those claim they are "true christians". It boggles the mind.