A Case of Conscience - James Blish

Father Ramon Ruiz-Sanchez S.J., is a part of a four man scientific commission to the planet Lithia, there to study a harmonious society of aliens living on a planets which is a biologist's paradise. He soon finds himself troubled: how can these perfect beings, living in an apparent Eden, have no conception of sin or God? If such a sinless Eden has been created apart from God, then who is responsible? Winner of the Hugo Award for best novel, 1959.