Joncnunn, as far as I remember when I obtained the enhancement one of the bonuses was for followers (including civs who had accepted my religion). I may be wrong of course.
You only get that one if the civ has NOT founded its own religion. To get it, a majority of his cities must have your religion as a majority religion.
Note that if your early to a religion and then spread it to him and then he founds one; then the positive modifier goes away. (In addition, if his capital was one of the cities you spread it to, then all your work in his capital goes away.)
On Standard map; 5 religions can be founded; so after all five have been founded, there are only 3 civs in which you can can this benefit. On some maps there may be one near you; but on others there won't be.
Also, a very common city state request is to spread your religion to them. Its worth a lot of free influence. And your not required to keep it as a majority, in fact if after completing it, some AI spreads his religion there, then the city state may repeat this same request.
Religious achievements:
You don't have to work to get "indoctrinated", that one happens if you forget to build an Inquisitor.
"Holier than thou" is easy; but always carries a diplomatic penalty.
"We are family" is bugged and won't unlock in single player; but will in hot seat.