Indeed, "Freedom of Religion" is a wonderful clause. However, it was written long before Joe Smith and the mormon religion, and it doesn't say anything about calling oneself "Christian", when - as mormons do - when they are not "Christians" in the slightest.
It says freedom of religion. The clause doesn’t say anything about Christian, Deist, Jew (all of which were practiced/believed in in America at the time of the writing of the Constitution) or whatnot. You may not like a person’s faith, but our constitution gives them the right to believe as they will...even if it’s not in the Christian sphere.
The country was settled in part by people who were discriminated against because they were shut out of things because they didn’t pass tests restricting who could hold public office and various other things because of their beliefs. Do we roll back the clock and the will of the founders to reinstitutionalize those policies? My Scotch-Irish and noncomforist ancestors would roll over in their graves and stand up and accuse me of dark things if I agreed to such ideas, particularly after they suffered under the same thing.
You may choose to make that a criteria for your candidate selection and urge other people to vote the same, but there’s no way you can say this should be legal requirement or policy.