A Case of Conscience by James Blish (Hugo Award 1959)
on Amazon USA:
I haven't read this one, but I've read the Cities in Flight quartet several times over the years.
The author of the Cities in Flight saga explores the conflicting demands of science, faith, and human feeling in this Hugo Award–winning novel.
Father Ruiz-Sanchez is a dedicated man, a Jesuit priest who is also a scientist, and a scientist who is also a human being. He doesn’t feel any genuine conflicts in his belief system—until he is sent to Lithia.
The reptilian inhabitants of this distant world appear to be admirable in every way. Untroubled by greed or lust, they live in peace. But they have no concept of God, no literature, and no art. They rely purely on cold reason. But something darker lies beneath the surface: Do the Lithians pose a hidden threat? The answers that unfold could affect the fate of two worlds. Will Ruiz-Sanchez, a priest driven by his deeply human understanding of good and evil, do the right thing when confronted by a race that is alien to its core?