from Michael Flynn's "Eifelheim"

"Eifelheim" was an interesting book describing a first contact with aliens in Medieval Germany, just before the Black Plague swept through. It plays with some ideas about light speed possibly not being the constant we assume it to be throughout the history of the Universe. (Which is a legit if unproven concept, and in the book also ties with time as being quantized, claiming that ties into some red shift quantization some have observed.)

The book really shows a thoughtful form of old Christianity, where "natural law" was explored as a way of discovering God's creation, and simplified "God just did it" explanations were not favored. But in some ways, us "moderns" have more in common with the aliens than with the pious of the time.
["There is nothing so well established as the constancy of light speed."] was the wrong thing to say, not only because there really were several other things better established, but because there is nothing guaranteed to get the back up of a no-fooling scientist than *argument from authority*.
Michael Flynn, "Eifelheim"

Think it through, Jackson. Light speed is frequency times wavelength. So if c is dropping and wavelength is constant, frequencies must be increasing. [...] So, atomic frequencies govern the rate at which atomic clocks tick. Of course, the speed of light has been constant since they began using atomic clocks to measure it. The instrument is calibrated to the thing it's measuring!
Michael Flynn, "Eifelheim"

"Those who hold the middle ground," said Gregor, "are often attacked by both camps. Between two armies is a dangerous place to graze your flock."
Michael Flynn, "Eifelheim"

"If a sinner truly repents, he dies to sin and a new man is born. That is what it means to forgive, for it defies reason to blame one man for the deeds of another."
Dietrich in Michael Flynn's "Eifelheim"