A Nestorian, to be specific.

He may have accepted Nestorian scriptures, but I'd argue that he followed the traditional shamanistic belief system of the Mongols.

It was the book "Genghis Khan: Emperor of All Men" by Harold Lamb where I read that claim. He may have been mistaken. I just read that it was the Kerait tribe who were Nestorian Christians. Their leader Togrul was a bodyguard of the young Temüjin, who eventually became Genghis Khan.

But it sure is interesting to ponder those connections! Thanks for bringing the history.