US Constitution, Article VI: “... no religious Test shall ever be required as a Qualification to any Office or public Trust under the United States.”
That clause allowed non-Quakers to run for federal office in Pennsylvania. It’s not a prohibition on private individuals making such decisions.
“Providence has given to our people the choice of their rulers. And it is the duty as well as the privilege and interest, of a Christian nation to select and prefer Christians for their rulers.” - John Jay
That’s about ballot access for candidates, not a rule binding voters. Voting is a First Amendment right. You can express your approval or disapproval of a candidate for any reason at all, be it policy, religion, or which way he parts his hair. You just can’t keep him off the ballot based on his religion.
God Bless America!