If you found a religion you should be prepared to defend it. If you find defending it to troublesome then don't both with religion. The AI will ultimately try to convert your cities. They can be weird about it - as some people mentioned they will sometimes just bypass your cities. I wouldn't rely on that though, because they will eventually try to convert.
Apostles with debater, Theocracy Government (+5 religious combat strength), there is a policy that also gives +5 religious combat, Inquisitors are cheaper and very good at defending religion. Unless you are quite sure your religion is not under threat of mass conversion always keep apostles/inquisitors around to defend. Moshka (Religious Governor) has some major religious defensive promotions if there is a choke-point city that regularly sees religious combat. To be honest though I rarely need to use him for that.
One other thing, while Gurus can heal I use them more for out of territory healing. You can heal religious units in a holy site you control in your territory.