FrDave of Blood of Prokopius can be a bit heavy on the Theology. But this post on the church and clergy is spot on and useful to just about anybody campaign.
I use the level equal rank in church rule myself with 3rd level being a full priest. The higher level you go the more your responsibilities become focused on the church as a whole.
Being a Catholic and a avid reader of history I read a lot about my faith. But I never picked up on the distinctions between the different orders (Deacons, Priest, and Bishops) that FrDave mentions. This will very useful in as a source of ideas for fleshing out some details of my Majestic Wilderlands.
In my oldest notes for the Majestic Wilderlands I made mention of three orders. The Order of the White Rod, the Order of the White Staff, and the Order of the White Quill. The Rods were administrators and parish priests, the Staff were mendicant friars, and the Quill were scholarly Monks. Color white was because Mitra symbol was a white lion head on a red field.
At that time Mitra was a man who was elevated to godhood. By 1990 I ditched that in favor of the current myth of the goddess Mitra and the Ghinorians her chosen people. I never did much with the orders since as they were part of the "old Mitra" but the notes are still around. When it comes time to do the various religions of my setting FrDave post is going to be very helpful.