"A Case of Conscience" is a 1958 science-fiction novel by James Blish, detailing with earth contact of a seemingly utopian society of marsupial reptillians on a planet with a very different geology and biology than earth, as well as the psycholoigical and even theological dilemmas posed by a different evolutionary track. The book is divided into two sections, one telling the story of an earth expedition researching the new planet, the other telling the story of what happens when they return to earth with a young member of the reptillian species.
The main protagonist on the planet is Father Ramon Ruiz-Sanchez, a Jesuit priest and biologist. There are three other members of the expedition, with different viewpoints, but the focus is on Ruiz-Sanchez. Relatively quickly into the book, he finds out that the reptillians reproduce in an unusual way: they release their eggs into the ocean, where they hatch and become fish, with ontogeny recapitulating phylogeny. I thought at first that the "Crisis of Conscience" was going to be him wondering how seemingly gentle aliens leave their young to struggle in the wild, but instead it makes him wonder if these aliens have souls, or have managed to form a rational society without any type of spiritual element. (And yes, this does seem like a bit of a jump). He becomes convinced that the planet is a creation of Satan, which furthermore makes him a heretic, because Satan can not create life. The second part of the book goes back to earth, where an egg of the reptillian gifted to Ruiz-Sanchez hatches and becomes a type of Antichrist, leading to the disruption of earth society of the time.
Sometimes when I am reading a science-fiction novel, I will read for a while, then flip to the title page and be surprised that it was written much earlier (or sometimes much later) than I thought. I did a double take when I found this was written in 1958, because it quite skillfully reconstructs an entire alternative society, from the basics of astronomy and geology. "Lithia", the planet of the reptillians, has a low amount of metals, and has a non-tilted axis in relation to its orbit, so it has little vulcanism and tectonic activity, and also no seasons or glaciation. How an intelligent, but very different, species could have developed, is described with great care, especially considering how much less was known about planetary formation and evolution in 1958. But after the book creates a believable planetary society, it then confuses the issue by making predictions about the future of human life that seem dated and quaint, with a humanity living inside of underground shelters. I never quite believed the basic idea that Ruiz-Sanchez would believe that a different race's biology or sociology would be literally Satanic. It seems quite a leap from "This race has a different biology and psychology" to "This is the devil's work".
As in many other science-fiction works, I would probably have to know more about the author's viewpoints to make sense of it: was he mocking the Catholic church, or was he himself making a conservative theological point? And, in either case, did he have an accurate view of how an educated Catholic theologian and scientist would view alien life? I don't believe he did. So, for these reasons, I agree with the write-up above this one: the first half of this book is a well-thought out and creative look at how astronomy, biology and psychology could all produce a sentient species that functioned very differently. The second half of the book has sociology and plotting that are much weaker, and much more arbitrary.