The grasshopper-looking aliens are mistaken by some for demons which gives the village a reputation for being cursed, especially after it's hit by the plague. As the "Carl Sagan meets Umberto Eco" blurb indicates, a lot of the novel is either 1. the aliens talking science and the priest talking theology, thinking they're having the same conversation, for example when it turns out the aliens are slowly starving to death because our food is lacking some essential nutrient:
“What was it Arnold [alien chemist] sought?”
The Kratzer [alien scientist] rubbed his forearms slowly. “Something to sustain us until our salvation.”
“The Word of God, then,” said Joachim [monk] from the fireplace.
“Our daily bread,” said the Kratzer.
Dietrich [priest] thought the concordance too neat. The words he heard the Kratzer speak were only those that the [translator] had matched to Krenkish clicks and hums. “What means ‘salvation’ to you?” he asked the creature.
“That we should be taken from this world to the next, and so to our home beyond the stars, when your lord-from-the-sky at last on Easter comes.”
or 2. comparisons between the organization of the alien society (a biological caste system, like insects) and feudalism/manorialism.
The aliens help defend the village from a nearby robber baron with their guns and jetpacks, and they help the villagers combat the plague with their knowledge of bacteria. They in turn are influenced by the heresies and peasant revolts of the time, concluding that though they find comfort in having their innate place in society, it is right to rebel against objectively unjust rule. It's a fun idea but the story is overly drawn out and there's only so much 'ah yes, brother alien, we all want to return to our home in heaven' I can take. Just a liiittle too cute. Then there were passages like:
[the priest, after an alien tried to explain binary code:] “Let us then use the term bißchen for this twofold number of yours. It means a ‘little bite’ or a ‘very small amount,’ so it may as well mean a small bite of knowledge. No one has ever seen Demokritos’s atoms, either.” The metaphor of a “bit” amused him.
Ulf had been working with a device that magnified very small things, by which Dietrich had named it mikroskopion.
There are several more of those (microbes, evolution, electricity, psychology, etc) and I found them eye-roll inducing. The whole book was teetering on this edge between heartwarming and clever, and tedious and cringe, tipping into the latter a bit too often for me.