Muslims are not allowed to join Christian churches. If he is a clandestine Muslim, I would think he would be spending time with some Muslims.
Only Hillary is doing that.
That could put all sorts of pressure on him as president. Think of the propaganda value to the various terrorist organizations. Do we really want to go there?
I think his current black-supremacist church is also a problem, because the line they spouted (until they changed their website) is creepily like Farrakhan's organization.
As for Hillary, I think her interest in Islam is purely financial (and possibly extracurricular).