It started with Reagan
An article at the History News Network website discusses the importance of religion in American politics. Not Afghanistan or Iran, but the U.S.A. The authors trace it back to Ronald Reagan in 1980.
The authors conclude with this:
This new age is one that many past presidents would hardly recognize. One can’t help but wonder what would become of a candidate today who, like John Kennedy in 1960, “believe[s] in a president whose views on religion are his own private affair.”

