Go ahead. Burn a bible (just make sure it's yours). Make a chocolate Jesus or a Piss Christ. Stomp on a crucifix. It will be a big yawn. So you'll piss off a few Christians - it still wouldn't get you dead. Or anyone else.
What's wrong with the idea that we should be blocked from saying or doing certain things that amount to symbolic speech is that it would spawn a crazy society.
I assume that a lot of people are very offended by misuse of their sacred symbols. The whole flag-burning case was such an example (such symbols may not be sacred due to religious feelings). If we adopt the idea that speech of which we do not approve should be made illegal on the grounds that it will offend someone so much that they will get violent, we'll be handing the likes of the Westboro Baptist Church members a fearsome weapon to control us all.
If the most crazy-mad person in the room gets to pick the dinner topic, the conversation's gonna get pretty weird very fast. And if the most crazy-mad person gets to say what cannot be spoken of, the conversation's going to be controlled by nutcases. The Koran is a book of poetry and a book of law, and the idea that Shariah represents is of the divine state - the community under Allah, living by Allah's law, and thus protected by Allah. To make it our law to forbid the expression of disrespect for that concept is essentially to make the US state enforce a type of religious proscription. I can imagine nothing more inherently subversive of our Constitution (which of course absolutely allows individual freedom to follow those beliefs).
The distinction - and it is a profoundly important one - is that I have the right to believe whatever I want, but I do not have the right to make you express respect for my beliefs, or forbid you to criticize or question them.
I'm going to quote from Texas v Johnson:
http://supreme.justia.com/us/491/397/case.html
Texas has not asserted an interest in support of Johnson's conviction that is unrelated to the suppression of expression and would therefore permit application of the test set forth in United States v. O'Brien, 391 U. S. 367, whereby an important governmental interest in regulating nonspeech can justify incidental limitations on First Amendment freedoms when speech and nonspeech elements are combined in the same course of conduct. An interest in preventing breaches of the peace is not implicated on this record. Expression may not be prohibited on the basis that an audience that takes serious offense to the expression may disturb the peace, since the Government cannot assume that every expression of a provocative idea will incite a riot, but must look to the actual circumstances surrounding the expression. Johnson's expression of dissatisfaction with the Federal Government's policies also does not fall within the class of "fighting words" likely to be seen as a direct personal insult or an invitation to exchange fisticuffs. This Court's holding does not forbid a State to prevent "imminent lawless action" and, in fact, Texas has a law specifically prohibiting breaches of the peace. Texas' interest in preserving the flag as a symbol of nationhood and national unity is related to expression in this case and, thus, falls outside the O'Brien test. Pp. 491 U. S. 406-410.
...
The restriction on Johnson's political expression is content based, since the Texas statute is not aimed at protecting the physical integrity of the flag in all circumstances, but is designed to protect it from intentional and knowing abuse that causes serious offense to others. It is therefore subject to "the most exacting scrutiny." Boos v. Barry, 485 U. S. 312. The Government may not prohibit the verbal or nonverbal expression of an idea merely because society finds the idea offensive or disagreeable, even where our flag is involved. Nor may a State foster its own view of the flag by prohibiting expressive conduct relating to it, since the Government may not permit designated symbols to be used to communicate a limited set of messages. Moreover, this Court will not create an exception to these principles protected by the First Amendment for the American flag alone. Pp. 491 U. S. 410-422.
We have historically not allowed people to make their own symbols sacred and binding upon the rest of society. And when we start to set up such a situation (at the time of this decision, 48 states had laws protecting the flag), we knock it down.