Since Sept. 11, 2001, two ideas about Islam have become axiomatic: that Americans need to know more about the religion and that "moderate" Muslims in the U.S. and other Western societies need to reclaim their faith from those who kill and maim in its name. With that background, it might seem churlish to cavil at a serious attempt to address both needs. But there is something about the PBS documentary Muhammad: Legacy of a Prophet (Dec. 18, 9 p.m. E.T.) that doesn't convince.
Muhammad's life--he lived from 570 to 632--is probably the best documented of any leading religious figure's before modern...