Go with everyone's definition of what a 'True Christian' is, and we have ZERO Christians.
Get the liars to respond truthfully when polled. That's an important bit. Because every statistic for 'belief' is at least doubled by people who don't want to 'look bad' to some stranger with a notepad. "Uh, yeah, I'm a good Christian an' I go to church ALL the time! Dat's da ticket!"