Prothero begins with an anecdote about an Austrian colleague who commented to him that while European students know much more than American students about the cultural history of religion, they do not go to church, while American students seemed to him much more likely to be church-goers, but to know next to nothing about the cultural history of religion. As Prothero states the problem: "One of the most religious countries on earth is also a nation of religious illiterates." 
By posing the problem as one of "religious literacy" (and "illiteracy"), Prothero very self-consciously puts himself in the line of E.D. Hirsch's book, Cultural Literacy, which came out in 1987. (Although that book is not online, the companion volume, The New Dictionary of Cultural Literacy, by E. D. Hirsch, et al. is online at Bartleby - and it includes a Bible section). I have not read Hirsch's book, but he apparently presents the thesis that American cultural literacy was undermined by the educational reforms of John Dewey. Prothero picks up on that argument, and is quite sympathetic to it. Here is what Prothero says:
"When I frst began teaching in the early 1990s I was a follower of Dewey and the progressives. [...] I cared about having challenging conversations, and I offered my quiz-free classrooms as places to do just that. I soon found, however, that the challenging conversations I coveted were not possible without some common knowledge - common knowledge my students plainly lacked. [...] In this way I became, like Hirsch, a traditionalist about content, not because I had come to see facts as the end of education but because I had come to see them as necessary means of understanding. This is a topic I'll come back to in separate posts. I started out my teaching career as a devoted follower of Dewey, and have become even more of one in the past years - but this does not mean I am uninterested in the problem of literacy. So, this is definitely a topic to return to in a later post.
Prothero then has to pose the problem of why, within the larger field of cultural literacy, religion deserves special attention. This is something that Prothero considers to be an objective issue, not purely one of personal choice: "Today religious illiteracy is at least as pervasive as cultural illiteracy, and certainly more dangerous. Religious illiteracy is more dangerous because religion is the most volatile constituent of culture, because religion has been, in addition to one of the greatest forces for good in world history, one of the greatest forces for evil."  Prothero notes again and again how pervasive religions has become in American politics and public life: "Ninety percent of the members of Congress, by one report, consult their religious beliefs when voting on legislation,"  he says, for example.
As someone who grew up on identity politics and the mantra of race, class, gender, I found this remark to be especially striking: "Religion is now emerging alongside race, gender, and ethnicity as one of the key identity markers of the twenty-first century." 
Throughout the book, Prothero will list many howlers where people in public pronouncements or in surveys of some kind demonstrate abysmal religious literacy. One of my favorite examples is this one: "A few years ago no one in Jay Leon's Tonight Show audience could name any of Jesus' twelve apostles, but everyone, it seemed, was able to list the four Beatles."  Now, I'm not so sure they could name none of the apostles, but I have no doubt at all that they were able to name all four Beatles. Everybody knows John, Paul, George and Ringo! Our heads are, indeed, full of facts - but they are facts that come to us from modern mass media, reinforced by consumer culture. As for the twelve apostles, do you want to try to name them? (Click here to see a list of the twelve when you are done.)
Prothero writes very eloquently about how the loss of religious literacy is a kind of national forgetting. In particular, it is an effect of the melting pot, in the worst sense of that word: "In conforming themselves to American culture, Protestantism, Catholicism, and Judaism had become little more than parallel paths up the mountain of the American dream."  For Prothero, this constitutes a kind of civic crisis: "You need religious literacy in order to be an effective citizen." 
Prothero admits that using the term "religious literacy" is a bit of shorthand. There are many religions and many literacies, too. He provides a thought-provoking list of the many kinds of literacies that could be taught: ritual literacy, confessional literacy, denominational literacy, narrative literacy and so on. (I'll have a great deal to say about narrative literacy in future posts, since that is my own interest both as a student of religion and as a teacher.)
Throughout the book, Prothero's choice of emphasis for both specific religions and specific types of literacy will be driven by the question of what he thinks people need to know in order to be more effective participants in American civic culture today. Hence the emphasis on Christianity, as that is the dominant religion in the world today. I noticed that in books reviews at Amazon, people were very disappointed in Prothero's emphasis on Christianity, but he is very clear from the outset about his reasons for this emphasis.
In addition, Prothero makes a strategic choice to focus on the educational changes that could take place at high schools and colleges in order to promote religious literacy: "The most effective way forward is to focus on secondary schools and colleges." . As a college instructor, this is something I think about a lot: what are the changes we can and should make to university courses in order to promote general cultural literacy and religious literacy in particular? What do I need to do as a teacher in order to tackle this serious problem? I'll have a lot to say about this in future posts as well, although for now I will just say that as someone who teaches at a research university where the professors are considered, first and foremost, to be scholars, I've concluded that there is nothing like scholarship to get in the way of basic literacy. :-)
So, those are the notes I had highlighted in reading through the Introduction to the book. I'll save my more detailed personal responses for separate posts later. Meanwhile, questions or comments here are always welcome - and I do highly recommend the book! It's still just out in hardback, unfortunately, but surely a paperback edition will be available soon. It's also available in audiobook form at Audible.com - which is how I happened to read the book in the first place. It's pretty rare for me to actually go out and buy a copy of a book after listening to it, but this book was so thought-provoking for me that I actually went out and got a hard copy, which is quite an endorsement in and of itself! I'll be tagging future posts on this topic with the label prothero, so if you want to see any other posts I've added on this topic, just click on the prothero label link.