Does anyone know the logistics of religious spread outside of using a missionary? I have a pair of examples; situation 1 I'm Gandhi on a standard size archipelago map/marathon speed/settler difficulty level/one city challenge. I monopolize every religion and yet they still manage to spread. I believe I sent the Confucian missionary to my closest neighbor but I put the Christian, Taoist, and Muslim missionaries to sleep in my city, and yet Judaism and Hinduism manage to spread around the globe. Situation 2 I'm Tokugawa on a standard size fractal map/marathon speed/prince difficulty. I started out on a small continent with only myself and Portugal who I eliminate before either of us found a religion. I have between 6-8 cities when I found Confucianism. By the time I get a monastery and my first missionary built, it has spread to all except my newest city. I must assume that trade routes spread religions to cities that haven't got one yet? If so... is there any way of predicting how the spread will operate?

religions spread naturally by trade routes. They are helped by open borders, and by the presence of a shrine. Also, a religion will only spread naturally to a city which does not have any.

There is a Article by VoiceOfUnreason (http://forums.civfanatics.com/showthread.php?t=237869) on the subject. Should cover all you want to know.