It's a good idea to get Yerevan (I think) as suzerain, then give the +20 combat promotion for your first 2 apostles. If you are invaded, stand next to a holy district, and let the others attack while you tank and heal pr turn. You can have them suicide en masse, and convert cities while doing so. Yerevan is also good for getting the natural wonder promotion, you can then convert 3 times, and return to envangelise a belief with further apostles.
The policies that increase combat powers are also good, sometimes important. The beliefs Denfender of the Faith, and the Crusader one, also give combat bonuses to faith units, and they can be good too. Basically your Apostles can become the invincible ones.