I don't know exactly what it does mean that more self-styled atheists than self-identified Christians know that Vishnu is a Hindu god, but I'm pretty sure it doesn't mean what a good many people want it to mean, which is that Christians are so by virtue of ignorance. Not knowing the meaning of Ramadan is problematic if one aspires to be a professor of religions, but it's no more problematic for the Christian than ignorance of fish-tank repair is for the architect.
To protest otherwise, to observe that the home and the fish tank are both dwellings, and that one can't really be certain of what he knows about a house unless he's also explored the fish tank, is to get the object of interest all wrong. The object of interest, when it comes to religion, is the truth of man and God. Having settled on the truth embodied for 2,000 years in the Christian faith, and promised for centuries before that in Judaism, the Christian need no more keep casting about for truth than one needs to consider all the other numbers before concluding that 2 + 2 = 4.
With that said, it's no secret that too many Christians know too little about our faith. The problem is not that Christians can't name the Quran as a Muslim holy book, or remember that Buddhists seek nirvana. The problem is that not enough of us can explain the Nicene Creed, or state why the Incarnation is central to our faith, or spot how often poor preaching is tied to an incomplete embrace of the great and beautiful scandal of our faith, which is embodied in Christ the God-man.
I believe, the more I delve into the roots of Christian faith, and the more I converse with thoughtful atheists, that some of the blame for the unbelief around us-among our neighbors, or among our own teenagers as they leave the Church-should be laid at our feet. Not fully knowing what it is we believe, we can't very well explain it to anyone else.
"Jesus died for your sins" doesn't stand by itself, after all. Absent an understanding of our dreadful sickness unto death, and an equal understanding that it was God Himself who crossed the divide for us, one doesn't really understand Christianity. And that is just the beginning, of course, because head knowledge is nothing (or perhaps even dangerous) without heart knowledge, which comes through prayer.
And quickly we've run far afield of the little Pew Foundation survey. Which I suppose is the point.