This is a great Sci Fi book, winner of the Hugo award way back in the day, that is unfortunately no longer in print.
The book is about Father Ruiz-Sanchez, a priest and a biologist, who is part of a UN commission to study the planet Lithia to determine its future. The Lithians are reptilian aliens that act like a perfectly moral race except that they have no conception of faith or belief--they are purely rational. This theological issue is the driving force behind the book.
In the end, I think the author shies away from making some bold claims about religion and human society in general. But still, it's well-written and a worthy read for anyone interested in Sci Fi. It really packs a lot into 181 pages.
I read this book because I reread the similarly setup The Sparrow & Children of God by Mary Doria Russell, some of my favorite books. Russell wrote a lot more about Jesuits in general and the Jesuits she sent into space. Russell's Jesuits' trips to Rakhat were very similar to Isaac Jogues (sp) trips to the New World. In fact, it could be said that Russell was writing historical fiction as well as science fiction. If you are interested I strongly recommend The Sparrow. My recommendation for A Case of Conscience is more tepid.
But you're reading this to find out about Blish. Blish writing in 1958 was understandably writing very much pre- Vatican 2. His priest does not question God. Or the Pope. Ever. This is a static book. It's a lot of talk, it's about ideas, they are very big ideas, but that has to be a kind of (science)fiction you are interested in. It's worth reading, but don't let this be the only sf you read.
It was interesting to read this book which was one of the first to take the intersection of relgion and science fiction seriously. The beginning and ending were good, but dragged in the middle. Unfortunately some of the theology didn't make sense to me, but maybe that is because I am Protestand, not Catholic.
I would recommend reading this, but think The Sparrow is the better book on a similar topic.
Iliked this book although it was a bit slow moving at times!