On Sunday, Nicholas Kristof had a column saying that Americans don't know much about religion. The two specific items he mentioned were "only one-third know that Jesus delivered the Sermon on the Mount, and 10 percent think that Joan of Arc was Noah’s wife." I can't find any record of the second question in iPOLL. There was one about the Sermon on the Mount, and indeed only 34% could answer correctly (it was free response, not a multiple choice question). But that was in 1954, so I hope that wasn't what he was citing. I did run across a question that has been asked a number of times since 1950: whether you can name the first four books of the New Testament:
Correct Partly DK or Incorrect
1950 35% 12% 53%
1990 37% 13% 50%
2010 45% 6% 49%
So there seems to have been a small gain in knowledge. Given the rise in educational levels, you might expect more. On the other hand, there has been an increase in the number of non-Christians and non-religious people.
[Data from the Roper Center for Public Opinion Research]