User blog comment:Fegaxeyl/Christian Mongols/@comment-1589986-20111006011843

This is mostly logic, as I'm too lazy to do extensive research. Four possibilities of origins of Catholicism, and possible explanations: 1) Catholicism came from Russia. If you can produce some group of Catholics that fled to Mongolia for religious purposes, you could plausibly procure something.  Think a bigger version of "Jewish Autonomous Oblast." 2) Catholicism came from China or Japan. Had the Spanish gained more footing in East Asia, they might have gained a powerful foothold. Not like the Spanish would conquer Mongolia, and even if they did, it would not stick, unless it so happened that the Japanese established some religious laws, or there happened to be a lot of Catholics in China. 3) Catholicism came from the Middle East. The Mongols absorbed Islam in their conquests in OTL.  This would require Catholicism to be a lot more present in the Middle East and Central Asia, which would require a complex POD. 4) Catholicism came from Tibet. This would have to require more creativity, but it isn't impossible, since Mongolia takes a lot of its religious cues from Mongolia.