The Pew Research Center just posted the results of their U.S. Religious Knowledge Survey--and the results aren't that encouraging. Atheists and agnostics actually scored higher than Protestants, who on average only got half the questions right.
Now to be sure, the questions cover more than just biblical knowledge. Evangelicals and Mormons did better than atheists and agnostics on the Bible questions, but they did not do as well on the questions relating to other world religions.
How well would you do? You can take the quiz by clicking here. Once you have taken the quiz, you can compare your results to the different groups who participated in the survey.