A Case of Conscience by James Blish

Winner of the Hugo Award •  The future of Earth will rely upon one man’s sense of right and wrong. . . . 

Father Ruiz-Sanchez is a dedicated man—a priest who is also a scientist, and a scientist who is also a human being. He has found no insoluble conflicts in his beliefs or his ethics . . . until he is sent to Lithia. There he comes upon a race of aliens who are admirable in every way except for their total reliance on cold reason; they are incapable of faith or belief. 

Confronted with a profound scientific riddle and ethical quandary, Father Ruiz-Sanchez soon finds himself torn between the teachings of his faith, the teachings of his science, and the inner promptings of his humanity. There is only one solution: He must accept an ancient and unforgivable heresy—and risk the futures of both worlds . . .