The Pew Forum on Religious Religion and Public Life released a survey on religious knowledge today. Atheists and Agnostics scored higher on it than anyone else, closely followed by Jews and Mormons, all Christians, Protestants and Catholics, were far behind.
That's overall, but when you get into specific religions it does show a startling lack of basic knowledge by practitioners. From the report:
More than four-in-ten Catholics in the United States (45%) do not know that their church teaches that the bread and wine used in Communion do not merely symbolize but actually become the body and blood of Christ. About half of Protestants (53%) cannot correctly identify Martin Luther as the person whose writings and actions inspired the Protestant Reformation, which made their religion a separate branch of Christianity. Roughly four-in-ten Jews (43%) do not recognize that Maimonides, one of the most venerated rabbis in history, was Jewish.
The study also showed that Americans have a fairly poor understanding of religions other than their own. Only about half of the people surveyed know that Martin Luther inspired the Reformation, the Dalai Lama is Buddhist, and Joseph Smith was a Mormon.