A Case of Conscience is a 1958 science fiction novel by James Blish. It won the 1959 Hugo Award.
In 2049, Ramon Ruiz-Sanchez, a Jesuit priest, accompanies an expedition to an alien planet to see if it should be opened to human contact. The inhabitants are a perfectly moral but completely non-religious people. This causes a crisis of faith for Ruiz-Sanchez, who comes to suspect that the planet and its inhabitants were created by Satan to trick humanity into believing that it can be good without God (that Satan can create relates to the heresy of Manichaeism).
A Case of Conscience is a relatively short novel, but the length never feels insufficient for the task. Blish does just enough world-building with his futuristic Earth to communicate the issues. Curiosity would have liked more information on the planet Lithia, but the story works well enough as is. Blish is heavy on the hard science here, at least enough to overwhelm (and thus fool) the casual reader. This is an idea-driven, not character-driven book, and Blish doesn’t bother with much deep introspection on anybody’s part (There’s great potential for it on Ruiz-Sanchez’s part, and it’s refreshing to see it not happen).
This novel was expanded from a novella, and there’s plenty of room left for further expansion. Egtverchi is a fascinating character and full of potential. But again, it’s refreshing to see the idea cow not milked to death – that was how people wrote fifty years ago, but not in our age of Robert Jordan and those hacks that cranked out the Left Behind series.
Like a good science fiction novel should, A Case of Conscience raises some interesting and valid moral and theological issues, and it’s entertaining, too.
P.S. - "A Spanish-speaking Jesuit priest travels to another planet as part of an expedition to make first contact with an alien race and subsequently suffers a crisis of faith" – this also is the premise of Mary Doria Russell’s 1996 novel The Sparrow. Whether Russell borrowed from Blish or not (obviously she did), Blish’s novel is incredibly superior (no great achievement, since Russell’s is so bad).