Ever since 1989, when the novelist Salman Rushdie found himself sentenced to death by Ayatollah Ruhollah Khomeini for the perceived blasphemy of “The Satanic Verses,” satirists in Europe have known that they were living on the edge of a volcano. Nervousness about mocking religious sensibilities has become more intense on the continent of Voltaire and Byron than at any time since the 18th century. That there are Muslims capable of taking murderous offense at blasphemy in a way that churches have long since outgrown is a reality that nobody who laughs at Islam can possibly forget. The cartoonists murdered Wednesday in the...