Ahem,
Back in the good old days of universal religion, at the very dawn of printed books, the best-selling category of books in Europe, other than bibles, were medieval romances. The works of Ariosto, of Montalvo, of dragons and griffins and enchanters and damsels in distress, and many of the damsels were not quite as well behaved as one would assume. All the things Cervantes made fun of (though he was a huge fan) a generation later.
The next generation of best-sellers were more “realist” picaresque novels that tended to even more racy material. Voltaire parodied this style (and a lot more of course) in Candide.
Why do you assume that in the “universal age of religion”, there were not numerous agnostics and atheists? Read Chaucer, Dante and others. Then as now secular leaders used religion to forge a common consensus and give themselves legitimacy despite their personal failings. Those who are adept at addressing the mystical needs of people usually prosper and rule.