Founders said in the Constitution “No religious test” shall be applied to holding office. You’re gonna have a problem with that and need an amendment.
In fairness he didn't say that Muslims should not be allowed to be president, he said he couldn't support one for president. A lot of people wouldn't support a Muslim for president...or a Mormon...or a Jew or Catholic or atheist. Personally I couldn't support a Christian for president if that Christian supported raising taxes and was squishy on the Second Amendment.
Since YOU get to elect the President, do you personally believe that an Islamic Koran Believing Muslim should be elected as President?
Would you not discriminate personally against someone who was a true believing Muslim when you entered the voting booth?
Do we need an Amendment to stop it, or simply an informed electorate?
Voters are certainly entitled to a religious test.