I think that's a key question. If they are getting 100g per city, that would be huge (assuming they actually do something constructive with all of that gold). Apart from that, there is no religious victory and if I knew I could spread like that, I would simply get faith to buy Great Persons. But knowing the AI, they're probably just spamming more prophets and missionaries, which are not cost effective.
Yeah I keep asking, because faith has been a very crucial catalyst for snatching games off the hands of other opponents in my recent MP games. Consider that on a huge maps if you proselytize the CS only we are talking about 20+ cities and the effect the RT or IP [God help you (pun intended ) if you face Byzantium and has BOTH] can have combined with strategic prophet bombings can make a religion snow ball in now time, earning you faith, gold, happiness GPs and whatever you invested to make those. I keep hearing people saying that Religion took a backseat in BNW, but in this posters humble opinion it has become a crucial utility to leverage my mid game strategy and push it to the finish.
Not that the AI can do this consistently, but I have nearly lost to Ethiopia once (King difficulty nonetheless) to a cultural vic (G&Ks) due to him having the number one religion spread (100+ cities) and each one of them giving him culture.... I remember Athens (holy capital of another religion) been flipped by 130 religious pressure.