We have no control over the other's clan decision to go christian or not, but sometimes it happens (maybe in 3 or 4 games that i played out of twenty-five/Thirty). But YOU can decide to go christian by accepting a trade agreement with the portuguese, buil a trading post, a church, then priests etc... Then send your priests on Ai clans territory and watch Your Priests get killed by the Ai Ninjas (or you can access your game files and watch all the cut scenes).