Religions that are nominally unified can mean lots of different things to different people, especially at different times in their history. You can see this in the history of Christmas. Conservative Christians of the past -- for example, both Oliver Cromwell and the Puritans during the 1660s -- banned the celebration of Christmas as part of their general dislike of fun. To Cromwell and the Puritans, banning Christmas celebrations didn't seem like a slight against Christianity. It was the truly Christian thing to do. Today's Christmas practices are those of a society where corporations spur us towards massive consumption. The mutability of religion is also evident in all the different forms of Judaism, with their different dietary restrictions and views of appropriate behavior on the Sabbath.
This is why I see discussions of whether Islam is a "religion of war" or a "religion of peace" as confused. (Over at Redstate, you occasionally see people arguing the "war" side and contemplating horrific evil as a result.) Muslims who feel peaceful will find some way to believe that peace is commanded by the Koran, and Muslims who feel warlike will find a religious justification of violence. The character of a religion is determined mainly by what its adherents feel like doing.
