Back in the mists of ancient time, I read Fatherland by Robert Harris, an alternate history where Germany won World War II. That was his first novel, wildly successful, and he's been prolific since then. I can't quite remember how this book got on my things-to-read list, but…
The book opens with a young priest, Christopher Fairfax, travelling on horseback through the bleak and muddy English countryside, on his way to Addicott St. George, a small village with its share of dark secrets. He's gone to bury the town's priest, who's died mysteriously. The book is set (Chapter One, Sentence One) in the "Year of the Risen Lord 1468". Hm, what's with the "Year of the Risen Lord" stuff?
Well, never mind, because things certainly seem medieval. When Fairfax arrives in town, he meets a bizarre array of characters. The dead priest's corpse is much the worse for wear, pointing to an unpeaceful demise. And he seems to have a stash of heretical texts and objects! Before you know it, Father Fairfax is being sorely tempted by passions of the flesh and mind.
Suggestion: don't read reviews, since they tend to reveal too much. You don't want to know about the (heh) insanely great plot twist at the end of Chapter Three.