As I recall (although the author of the comic did not cite it and I cannot find it now) the poll referred to was a Pew Poll that has been widely misinterpreted. My recollection is that those polled who identified as atheists or agnostics also tended to report growing up within more conservative Christian contexts. Thus they received their biblical training within the Christian context. On the other hand, there are many who identify as Christian on polls who are not terribly serious about their Bible learnin’. Still, our congregations are woefully ignorant and the comic is funny.