Congressman Keith Ellision of Minnesota took a ceremonial (not official one) oath on the Islamic Quran. He is a Muslim. Many conservative Christians were upset that he did this. The qustion is, why?
It is simply because many conservative Christians believe that the government and the US is an arm of their church. They literally believe that we are a Judeo-Christian republic as founded by Jehovah and Jesus. They believed that the Puritans and not deist founders such as Thomas Paine, James Madison and Thomas Jefferson founded the United States nation and government. The Puritans were early theocrats to the US. They did not favor religious liberty for anyone but themselves.
Obviously many of the founders in the 1770s/1780s were devout Christians but all the Founders were children of the Enlightment period. Peter Starks is an atheist or at least nontheist congressman, should he have to put his hand on the bible? Many Christians seem to believe that there is an absolute obligation to use the bible if one takes an oath of office in the US. That if you dont' like it you should leave.
But again, we aren't a theocracy. No freedom of speech or free exercise of religion exists in the bible. Indeed, the bible is strongly anti-free exercise of religion if one practices a religion outside that of the Israelite and their deity. Then, the penalty for worshiping another deity (especially for an Israelite) is death.
A second reason is that some Muslims are reactionary violent Islamists but the obvious point is not all of them are that. Keith Ellison is not going to strap on a bomb and bomb congress. If we want to appeal to the Islamic world then saying that a Muslim congressman can't use their bible in taking a ceremonial oath does not exactly build bridges, to say the least.
Personally, if I was elected to public office I would choose the Constitution because that is what congressmen defend. If not the constitution, then Paine's, "The Age of Reason."