Some of what we don't know is comical (Who was Joan of Arc? Noah's wife, answered 10 percent of Americans in one poll). And some of what we don't know is surprising (Who said "Blessed are the poor in spirit, for theirs is the kingdom of heaven"? Fifty-six percent of people who identified themselves as born-again Christians didn't recognize it as a quote by Jesus from the Sermon on the Mount).

Add up all of what we don't know, not just about other people's religions but about our own, and it makes America "a nation of religious illiterates" — at the same time that we profess to be one of the most religious countries in the world, says Stephen Prothero.

Prothero is a Boston University professor, chairman of its religion department and author of "Religious Literacy: What Every American Needs to Know — And Doesn't." The book, published this month, includes a small encyclopedia of religious information, from al-Qaida to Zionism. The heart of the book, though, and the reason it's getting lots of attention (including a coveted spot earlier this week on "The Daily Show"), is the light it shines on American ignorance when it comes to the who, what and why of religion.

Prothero is unflinching in his criticism but hardly self-righteous. Here, for example, is a description of the day when his youngest daughter received a new Bible at the Lutheran Church his family sometimes attends: After the service, Prothero asked his daughter, then 8, to name one of the books of the Bible. No response. "'How about a Bible character?' I asked somewhat desperately, 'other than Jesus?,'" Prothero writes. "To which, after giving the matter considerable thought, she responded, 'Tom."'

In "Religious Literacy," Prothero outlines how it is that Americans have become so unknowledgeable about religion and offers a far-reaching "modest proposal" to turn things around: the re-introduction of religious education in public schools. But before you either applaud or scream (depending on your world view), understand that Prothero is not trying to make students more religious.

It's all about prepositions: not of religion but about religion. No preaching, just teaching.

That's not unconstitutional, he points out. But the current strategy of trying to abide by the First Amendment by avoiding teaching about religion in public schools "may well be violating the Constitution, by indoctrinating students into a secular view of the world." At a minimum, he says, "this tendency is fueling the self-identity of many evangelicals as a besieged minority."