I got most of the way through this book a while back but had to return it to the library. Now I can't remember the title or who wrote it. I do remember it was written sometime around 1955.
An expedition of ~5 humans has been sent to a jungle planet, the first intelligent species discovered by humans. These people are reptilian, and live without government in a highly cultured self-organizing society: adolescents have an instinct for picking their vocation. The life cycle of the lizards involves laying tons of eggs in marshes; tadpoles hatch and the juvenile/adolescent lizards have a hard time of it -- most of them don't survive to adulthood. Those that do never know their parents.
The purpose of the human expedition is to decide whether the planet is safe for human visitation.
The main character is a Jesuit (iirc, certainly a Catholic), who is both a botanist/doctor (can't recall) and the spiritual adviser to the trip, who befriends one of the Lizards, a dignified and kind "man". The Jesuit comes to the opinion that the Lizards, who have nothing resembling religion, nor do they love (or even know) their children, nor do they marry, are sent by Satan to tempt humanity to a loveless and secular lifestyle.
The last part of it I read was where the Jesuit was having a debate with the other members of the expedition as to whether to recommend that the planet be closed off to all human visitors.
Can you guess what book this might be?