Apples-oranges. Mormons are allowed entry. They are not allowed to practice  polygamy. 

Muslims are allowed entry (or should be). They are not allowed to stone atheists like me to death. We don't get to exclude people based on religion, we are allowed to criminally prosecute based on religion-based illegal behaviors. "Faith healing" is an example of illegal behavior on a Christian side - it's considered child endangerment. But we don't restrict Christians from entering the country, nor should they. People of all religions are allowed to be given equal protection when applying to the US, but they must adhere to our laws.

If they apply to come here with intent to break the law, then that is a valid reason for their visa to be denied. But denying based simply on religion is not Constitutional.