A recent Pew survey revealed that Americans are rather ignorant in regards to religion. Interestingly, Catholics did the worst while atheists and agnostics did best in terms of what they knew.
The finding about ignorance matches my own experience. When people find out that I am a philosophy professor, the talk often turns to the matter of religion. When I was younger, I was rather surprised at how little people knew about even their own professed faiths, but I soon came to expect this as the norm. While I do expect this, it does bother me that so many people do not seem to know what it is they strongly profess to believe. What dismays me even more is the fact that people are generally not inclined to correct this ignorance nor often interested in subjecting their beliefs to some critical thought.
I am not surprised that atheists and agnostics know the most about religion. One reason is that atheists and agnostics are often (but obviously not always) reasonably well educated. As such, they would tend to know more about religion. Another contributing factor is that some people end up as atheists or agnostics after trying out various faiths-hence they will often have a broader base of experience to drawn upon. A third factor is that people who are more critical and inquisitive will often tend towards atheism and agnosticism and such people are more likely to know more about various faiths. A fourth possible factor is that a broader knowledge base actually tends to lead people towards atheism and agnosticism. That is, perhaps atheists and agnostics believe what they do because they know more about religion. After all, seeing so many people sincerely devoted to beliefs that are at least inconsistent with each other would incline a person towards skepticism about religion.
Of course, I have (anecdotally) found that it is not uncommon for atheists and agnostics to be ignorant of the philosophical arguments for God’s existence. However, most folks are ignorant of these arguments.
In terms of what religious people would tend to know less about other faiths, one obvious reason is that if someone thinks they have the correct answer, there is little reason to learn about other faiths. Some might be inclined to say that people of faith are more likely to be ignorant and lacking in intellectual curiosity (or vice versa). This is, perhaps, a possibility.