A couple of quotes. The first from the excellent contemporary scholarly book called Muslims: Their religious beliefs and practices by Andrew Rippin (don’t let the title fool you — he delves deep into the various elements that have combined to form Islam from its pre–history to the present day and includes current scholarship on Islam, challenging traditionally–accepted views on Islamic ‘history’):
While Muslims may think those who deny the existence of God or who utter blasphemies about Him are misguided, such discussions will not offend in the same manner in which discussions over Muhammad will. Those who insinuate evil of Muhammad or who cast aspersions on him are considered to be insulting Islam. This, upon consideration, is not surprising.
I know this myself, from having grown up in an Islamic community. It means muslims put Muhammad at the centre of Islam, not Allah! And the reactions Rippin notes above made me recall a passage I read in an autobiographical work by Ziauddin Sardar, a well–written and witty, not to mention interesting, book called Desperately Seeking Paradise. In this extract Sardar details his reaction to reading Rushdie’s The Satanic Verses:
Rushdie had plundered everything I hold dear and despoiled the inner sanctum of my identity. Every word was directed at me and I took everything personally. This is how, I remember thinking, it must feel to be raped… The life of the Prophet Muhammad is the source of Muslim identity. Muslims do not merely emulate his character and personality and follow his sayings and actions: it is the Prophet Muhammad who provides them with the ultimate reason for being a Muslim.
I do wonder if he or others would react like this if someone spoke out against Allah as opposed to Muhammad. Presented for your consideration: the reaction of muslims worldwide to the Danish cartoons (nobody lampooned Allah, only Muhammad), and the case of the Teddy Bear Named Muhammad (I recall some sources referring to this as ‘blasphemy’ — surely only a crime against God ranks as blasphemy?).
because for every death seems the world makes muslims responsible 
