I'm not surprised by any of this. Religion is an inescapable part of art history, and I end up teaching a lot of it in my art history classes (all except modern art, significantly). I am always amazed by how little my students know about the subject, and what little they do know, they learned from TV and the movies, not from their churches. It also doesn't help that the public discussion of religion is entirely dominated by fanatics of one kind or another. Every year, I teach classrooms filled with Christian students who do not know the difference between Protestant and Catholic, who've never heard of the Reformation or Counter-Reformation, who've never heard of St. Francis, Martin Luther, John Calvin, or Ignatius Loyola. The students I've had who appear to be most informed about their own faith are usually Muslims. Most of my students are either Catholic or evangelical with a few Muslims. I used to get a wider religious diversity out in the suburbs with a substantial number of Jews, Sikhs, Hindus, Buddhists, and secularists. So far, the Bronx in my experience is religiously a little less diverse, though I get the occasional Wiccan student.
I expect a certain measure of religious ignorance from my evangelical students. Evangelical Christianity does not exactly encourage intellectual activity, and tends to be ahistorical. I'm a little surprised at the ignorance I see in my Catholic students. The Catholic Church has (or had) a vigorous intellectual tradition and used to be very accomplished at producing a large population of religiously literate believers. My Catholic students are no better than my evangelical students when it comes to religious literacy, including about their own faith. I'm always surprised at their surprise when they find out that it is the Pope that has the last word in their church, and not their personal consciences. I'm guessing that American Catholicism is taking a lot of its lead in religious education from the evangelicals, who do not value knowledge, critical thinking, or reflection very much.
We are in the middle of a huge cultural conflict for which we are entirely unprepared. In these matters, our prosaic nature as Americans has not served us well at all. In my experience, most people don't know much about their own cultures, or care very much about them, unless they perceive their culture to be under threat. And yet, like it or not, we live in a cosmopolitan world where negotiating across cultural and religious differences is a necessary skill for survival. Even the Amish and the Hasidim recognize this.