This guide will tell you how to completely control Catholicism, making it so that only you have Popes, so that you cannot be excommunicated, so that you can call crusades wherever you wish (out of the preset targets), and so that you cannot lose Papal favour.

For Step 1, you simply need to start a new Campaign as a Catholic Faction - not as the Pope, that's the easy way, which involves coding and can be glitchy-.
Factions which you can pick from include:
-England
-Scotland
-France
-The Holy Roman Empire
-Poland
-Portugal
-Hungary
-Spain
-Milan
-Sicily
-Venice
-Denmark

For this step, it is recommended that you either choose Venice, or choose Sicily. You will see why later. Venice is mostly the better option of the two, else you will likely have more enemies than needed (The Moors do not matter).

If you chose Venice or Sicily, great, Step 2 should be easy. If not, then aw, Step 2 will take a bit longer than otherwise needed, especially if you picked Portugal or Scotland.

Venice or Sicily:
a) -Take the Rebel settlements you are near to, start off the first 20~ish turns how you normally would, though with Sicily try to avoid expanding towards Egypt, instead considering taking out Milan and Venice, whereas with Venice take out Sicily and some of The Holy Roman Empire's cities (but not all, you don't need all, it's a waste of time for now). If this took longer than 20~ish turns then you took too long, you want to finish this guide before The Mongolians arrive, not after The Aztecs discover gunpowder.
b) -By now you should be excommunicated. You should have no problems with this step due to that, other than the fact the entire Catholic section of the world will declare war on you soon enough. Attack The Papal States and capture Rome, if you did well, they should not have any other cities.
c) -Continue killing every Pope that gets elected, upon the turns they get elected. Repeat this to avoid a Crusade being called upon you, or upon the Islamic Factions (they will be safe, hence will gather more soldiers to keep the Mongolians at bay for longer, just in case you're still not ready after possibly failing a's time limit).

The other Catholic Factions:
a) -Take the Rebel settlements you are near to. Swiftly begin conquering nearby Catholic Factions along the way towards The Papal States (e.g.1: Scotland would conquer England, then France, then The Holy Roman Empire and Milan, then you should border the The Papal States., e.g.2: Portugal would conquer Spain, then France, then Milan, then you should border The Papal States., e.g.3: Poland would conquer Hungary, then The Holy Roman Empire and Venice, then you should border the The Papal States.). Try to complete this step within 50~ish turns, at most.
b) 'b' is the same as the 'b' for Venice and Sicily.
c) 'c' is also the same as the 'c' for Venice and Sicily.

There should no longer be any way for Crusades to be called by the other Catholic Factions, nor any way for the Islamic (and other) Factions to ally the Papal States. By this time every Catholic Faction near you should have declared war, if they have not done so yet, they eventually will.

Now:
a) Begin building up a military, if you haven't got one yet. Prepare it so that it can fight at least one third of the factions, try to finish this step within at most 5~ish turns, depending on previous performance.
b) Begin conquering every Catholic Region on the map. Try to finish this by turn 80, at the latest.
c) When you have conquered every Catholic Region on the map, this Step is finished. Return to the guide then.

Now, you control almost the entire world, basically. With every Catholic Faction other than your own eliminated, it is now time to rule Catholicism:
a) Begin recruiting priests. As there is no Pope, and you rule either Portugeese lands (if you didn't start with them, they will likely be Islamic) or Hungarian lands (again, they will likely be Islamic), the Priests should have a chance at gaining 5 faith and becoming a Cardinal. When they are dubbed Cardinal, they will instantly become the Pope.
b) One of your Priests has become the Pope, and likely many others have become Cardinals, you now also have peace with The Papal States and very high influence, and every possible crusade target will have almost or exactly 100% followers of it's own religion due to the Catholic AIs being focused on you. Offer an alliance to the Pope, and then request a Crusade upon whomever you want out of whom you can, for extra experience and extra florins, along with Chivalrious traits to help you with keeping the public order.

Now, there is nothing any of the AI factions can do, with you being the only Catholic Faction, and The Pope being trapped inside your lands with no access to the other factions diplomats. You can call a crusade upon any of the preset targets, and finish world domination to complete your Long Campaign. By this point you already have more than 70 settlements, so it should be easy to finish the last 40 or so.
Use this guide wisely and have a good day. Also keep in mind that it's an utter waste of time to do all of this, since Long Campaigns only require 45 regions, with one of them either being Jerusalem or Constantinople (in the case of the Catholic Factions). Most proper Campaigns will never get far enough to even have the Mongols spawn, nor the plague. This holds no benefits if you're good at M2TW, but it is a challenge to take, nonetheless.

-=Thus ends the guide=-