Islands in the Sea

Islands in the Stream is a short story written by Harry Turtledove.

CAUTION SPOILERS

In Islands in the Stream, Constantinople and thus the Eastern Roman Empire fell to Muslim invasions, leaving the Western Roman Empire without a buffer state.

Centuries later, Muslim and Catholic emissaries visit a heathen king whose land is between both empires, in an attempt to convert him to their respective religions. After much prosyletizing, he converts to the Muslim faith because the Caliphate commands a stronger empire than the Pope. The participants realize that Christianity is doomed to remain in an isolated part of the world without a powerful empire supporting it.