I think the idea is that atheists and members of certain other faiths know more about religion in general than Christians do.
In short, many Christians are kinda ignorant. Yeah, mean, ain't I

.
There is a possible explanation for this phenomenon. Traditional backwoods (being stereotypical here) Christians are often raised in their religion and in its surrounding culture and generally taught the attitude that messing with other religions is taboo. So, in the worst case scenario, they don't know much of
anything, save that
particular point of view that they are taught. Not just talking about different religions here, I'm talking about denominations or subsets of doctrines in denominations! It can get pretty ridiculous! Got a relative that is stuck on ol' King Jimmie-- doesn't even have a decent explanation for some of the stuff he believes like that. His attempts, if you call it that, at an explanation will blow your mind, possibly laughable for a rational human.

More or less just repeating the argument. Nothing... Zilch... Well, I guess at least he's going to Heaven.

I wonder if some Christians can even answer most of the Christian (especially protestant doctrine/raw Biblical) questions. I'd be curious on those statistics. Man if they were poor, it'd be pretty ridiculous.
Christians aren't the only ones who don't mess with other faiths. A devout Jew (such as Orthodox, not Messianic) may consider the New Testament to be taboo from what I've heard.