By now, you might have seen the results from the Pew Research Center on U.S. Religious Knowledge. The results aren't pretty. Of 32 questions asked, the average was 16 correct answers.
We are woefully ignorant about religion, our own included. Half of the people surveyed could not even name the four Gospels.
You can read Rev. Chakoian's thoughts on why this is the case here.
I would offer a different explanation: When a religion emphasizes faith over knowledge, only the most intellectually inclined and competent of its adherents will bother to learn much about it themselves. The rest are content to to accept on faith what they are told is necessary for salvation. This latter group, representing the vast majority of the faith, are therefore mostly ignorant (and contentedly so). We can't expect them to do well in a survey like that conducted by Pew.
Furthermore, among the minority of intellectuals who do pursue a more complete understanding of their religion, many will develop information that causes them to question orthodox interpretations. After all, the orthodox interpretation is not self-evidently the most reasonable or likely one. Orthodoxy is accepted on grounds of faith, not reason or experience or knowledge. Thus, many of the intellectually inclined will end up leaving their religion, perhaps exploring others, or even becoming agnostics or atheists (and thereby further shrinking the ranks of the knowledgeable faithful even more).
In my mind this explains why agnostics and atheists did so much better than the faithful in the recent religion quiz sponsored by the Pew forum. The startling conclusion that one can draw from the Pew survey results is one that not many are mentioning: The more educated one is about religious matters, the more likely one is to be an agnostic or atheist. In my mind, this, more than anything else, explains why the faithful are on average so startlingly ignorant of their religion: Religious leaders educate their followers at their own peril!
That's not to say that there are no educated persons among the faithful. There are. And many great ones. But such educated faithful are far rarer than they should be.
You can take a sample quiz and test your religious knowledge here.