The creation of the twain, Fafhrd and The Gray Mouser, would be sufficient glory for most any writer. But Fritz Leiber labored long in the genre gardens. Among the other fruits of his mind was the science-fiction/quasi-horror novel, Gather, Darkness!
Set in the future, the novel posits a world ruled by an ecclesiastical caste, known as the Hierarchy. The church runs everything in a strict, authoritarian manner that Mao or Stalin could only have dreamed of. The mass of humanity is peasantry, living a feudal serf level existence. But the book is not a simplistic attack on religion. There are twists.
Twist the first: The Hierarchy are knowing, intentional hypocrites. The Hierarchy was formed long ago by scientists who believed they knew best how to govern humanity in the best interests of all. (Sound familiar?) The miracles, beatific emotions experienced in church, religious manifestations, etc. are all scientific trickery.
Twist the second. The Hierarchy has opposition: witches. But as the Hierarchy is science disguised as religion, the witches are science disguised as black magic. But most of the Hierarchy have trouble truly accepting this Having subconsciously absorbed the veracity of the religion that they falsely preach, they subconsciously believe the reality of witchcraft and the dark hand of Sathanas.
Leiber writes some memorable characters into this. He reveals matters in his own, finely judged time. Secrets and backstories are hinted at, then explicated as needed. We have heroics, self-sacrifice, surprise reveals, capable villains, and sympathetic heroes put through the grinder. Good stuff. Give it a try if you haven’t already read it.
Something else you ought to give a try is Cesar the Bravo. If you’re a fan of Fafhrd and The Gray Mouser, you’re likely up for some swashbuckling, Sword-and-Sorcery adventure. In that case, you’re in luck; Cesar’s your man.