What if Judas didn't do it?

[Belacqua]

Which christians believe that the entire Bible is myth?

I understand that in conversation, calling something a myth can mean that it's not true. ("Russian influence cost Hillary the election!" "No, that's a myth.") 

In literature, in Bible hermeneutics, the word has a different meaning. 

A myth is any narrative that functions as a myth. It may have accurate historical elements, or none.

For example, among a lot of post-Dawkins Internet atheists, the stories of Galileo, Giordano Bruno, Hypatia, and the Library of Alexandria function as myths. The stories contain elements of truth. They refer to real people. But they are myths because they are repeated in order to make ethical and ideological points. Often they are embroidered to the point of being fictional, or trivial elements are emphasized in order to make events have a meaning which would have astonished people who were present at the time. I've heard people say that Galileo was tortured, that Bruno was executed for rejecting geocentrism, that Hypatia was something like a modern scientist, that the Library of Alexandria was a repository of scientific information burned by Christians who hate science. All of these things are wrong, but help to emphasize the meanings that certain atheists want to convey. 

So the stories in the Bible may contain more or less historical truth and still be myths. The authors of the Bible use the stories to convey ethical and spiritual messages. That is their purpose -- not accurate journalistic reporting. 

To answer your question, I'd say that all Christians believe that Bible stories are myths. Because the stories are not simply an account of events, but narratives intended to convey important messages. They may also be true as history (or not) and Christians will vary on how much they take to be true in this way. Some Christians insist that to be meaningful a given event in the Bible must also have happened historically, and some don't.