Should take a look at how state level laws and regulations are formed :p
The argument is not for combining church and state but rather allowing people to vote by their own beliefs. Sharia law would def be a combination of church and state. Not to mention a majority of americans being muslim is bordering a laughable hypothetical. Sure all the muslims in america could move to a single area and then influence that particular area's politics but as with North Carolina you would see resistance and pressures from other states and the federal government.