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.