A stupid idea
Setting aside, for the moment, the xenophobic nature and the unconstitutionality of Donald Trump's proposed ban against Muslims entering the country (just for the moment, since those issues should not be permanently dismissed), the proposal has some practical obstacles to implementation.
Strict enforcement would require the closing of some embassies in Washington, D.C., and the exclusion of diplomats from the United Nations headquarters in New York. Even if the administration wanted to make exceptions for diplomats, many diplomats would likely refuse to enter a country that discriminates against Muslims.
Foreign countries would likely reciprocate against Americans, including diplomats, tourists and business people. Engineering and manufacturing firms in America would see some of their business go to competitors located in Europe, Asia, Canada and Australia.
The reaction would be immediate from Islamic theocracies, but other countries would join in some level of sanctions against America as a result of religious discrimination.
Another question - how would a ban against Muslims be enforced? Religion is not listed on passports. Trump suggested that the custom officials ask the person trying to enter the country.
If it's that easy, why not simply ask, "Are you entering the country to commit terrorism?"