This was obviously a mistake on the part of the missionaries!
They nearly made the same mistake in Russia and Ukraine ~ still, it was common for Mongol overlords to become Christian. The Mongols in Estonia and Finland were also frequently Christian, and I would suppose some of them were still around when the Protestant Reformation took hold throughout Scandinaia.
(Hint: I know a bit about the history of the peoples who speak Uralic-Altaic languages, or the apparantly related Finno-Ughric languages. A neighbor girl affiliated with a church in Northern Virginia traveled to Mongolia as a missionary and helped found the first Christian church ever approved, authorized or recognized by a government in Mongolia!)
(Note: A few years back National Geographic featured the Mongol Conquests. They featured Ghengis Khan's Christian grandson).
Then choose a side and enlist with us. This is war...