Quick lesson on the Constitution:

"No religious Test shall ever be required as a Qualification to any Office or public Trust under the United States."

Source: Archives.gov.

This is a restriction on government, not a restriction on voters or a restriction on what people are allowed to say.

The government may not implement a religious test as a matter of law, but voters can vote for or against a candidate for office for whatever reason they want.

Therefore, the argument that anyone is acting an an unconstitutional manner by saying he would not support a Muslim for President is completely wrong.