I feel like there must be good books about religion in space but I can't think of any offhand.
Phyllis Gotlieb's science fiction frequently has religion in it, human as well as alien, but I am not sure that any of her novels specifically focus on it as opposed to taking it as a part of culture. (Some of her shorter fiction, yes: mostly Judaism. Her novella "Son of the Morning," later incorporated into her novel A Judgment of Dragons (1980), is one of the great pieces of Jewish science fiction, especially since to one of the characters it looks like the Yiddish fantastic.)
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Phyllis Gotlieb's science fiction frequently has religion in it, human as well as alien, but I am not sure that any of her novels specifically focus on it as opposed to taking it as a part of culture. (Some of her shorter fiction, yes: mostly Judaism. Her novella "Son of the Morning," later incorporated into her novel A Judgment of Dragons (1980), is one of the great pieces of Jewish science fiction, especially since to one of the characters it looks like the Yiddish fantastic.)