You needn't bother---the Founding Fathers understood that religious tests were complete crap over 200 years ago.
If someone still fails to understand that such a practice is an odious affront to our Republic, then they are probably immune to reason.
However, reasons why an INDIVIDUAL will vote, or not, for a candidate are his own business. I won't vote for example for someone--if their office has any impact on the subject--who is not pro-life. Is this a kind of "religious test" on my part? Of course it is.
Is my choice a "religious test" as meant by the Constitution or the Founders? No, of course not--they meant something written into law that says a candidate must be say, Episcopalian, to run for office--THAT is a religious test. If I made a choice, my personal choice, only to vote for Episcopalians though (foolish as that may be--and likely always contradicting my pro-life choice) that does not constitute a "religious test" as meant by the Founders.
So, please don't accuse those of the many of us who won't vote for atheists as somehow being unconstitutional in demanding a religious test....that's just rubbish.
We have an inalienable constitutional right to vote for whomever we wish for whatever reasons we wish--even (or especially) if those reasons are religious.