Read the novel “The High Crusade” by Poul Anderson.
“a hundred races, united in Christendom, the English tongue, and the English Crown...”
“they proved they had souls, and could be saved...”
“our former enemy Hurga, now known to this world of Yorkshire as Archbishop William...”
One of the later subplots in the novel is the wholesale conversion of multiple pagan or atheistic alien races to Christianity.
Coolest post of the thread. Poul Anderson rules, he is always involving historic Catholic and Orthodox elements into his stuff. Gene Wolfe also uses Christian imagery/aspects into his speculative fiction/sci-fi/fantasy. And of course there is CS Lewis. All three treat Christianity fairly in my opinion, which is pretty rare in my experience.
Freegards