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.
In part it is because most of the “fantasy” fan base of the time was deeply religious. Whether they were moral people or not is another question of course.
St. Teresa of Avila, the mystic, was a notable fan of these epic romances.
I can imagine St. T at a modern anime con.
Cervantes’ anti-hero Don Quixote is immediately translatable to modernity. Imagine an old retired guy in Modesto, CA, with a tremendous comics collection, who one day goes nuts and imagines he’s a superhero.