Wow, am I the only one who thinks, that spreading the religion with the second prophet is on many occasions a better choice?
I usually take Tithe and therefore benefit strongly from my religion in enemy cities. Therefore expanding my religion the fastest (in most cases before they have their own religion) to get the early pressure without foreign religions declaining it is the way to go. Of course I'm aware, that I can loose some of the better follower beliefs that way, but usually there will still be at least one worth taking. Spreading the religion early to 4 enemy cities gives you more than adopting Itinirent Preachers earlier, but you have to choose the cities wisely (not necessarily ones next to each other so that they apply pressure on a larger area). Additionally you don't get negative modifiers from civs from expanding your own religion, since they do not have theirs yet. A religion spread that way will be extremely difficult for the AI to get rid off, and expands later by itself, and such a strategy won't work with missionaries, cause they aren't as strong as prophets and have attrition. You'd need 3-4 missionaries to get as much religion spread as one prophet so in faith cost and mostly time it is a better option (for such a strategy of course, I can think of other strategies where enhancing would be better).