Whichever catholic faction you are, you'll only expand so much into Europe before everyone will gang up on you. A typical example is the english campaign I'm in now. I crushed the french, who had no allies from day one, early and took all of their territory and Navarre from the rebels. Assassinated the Aragonese and bribed the resulting rebels there. Bribed rebels in Burgundy and everything was fine still allied to virtually every faction. Then the germans invaded... Held those off without upsetting the pope. Everyone stayed allied to me, germans lost alliances, then the Italians had a go. Few years later the spanish. The Spanish and Italians got lucky and decimated half of my fleets and then the Spaniards defeated me in Aragon and Navarre. That triggered a massive civil war followed by a massive civil war. I've pulled back to the coastal province and have just re-expanded further inland.

It seems to me that the AI attacks once you get so many provinces? Or that the AI always plays a GA game and considers many provinces as being theirs by right. I had never made war on any of them before they attacked me and had only bribed or taken rebel provinces. All of their attacks broke alliance treaties and if the spanish hadn't attacked me as well I'd have easily destroyed the Germans and Italians who were suffering unnumbered civil wars and revolts.

Through all this neither the Spanish the Germans nor the Italians got excommunicated, though I did, despite having me under siege for over two years in toulouse and repeatedly attacking my provinces and shipping, while I didn't attack theirs once.

So it seems you can't play the popes' game and use him to your advantage and the AI catholic faction can do what it likes so long as it doesn't attack the pope or his allies?