An easy way to secure your borders permanently and placate the pope permanently is to gift regions you conquer to him. Depending on how you wish to play, after all the provinces would take away from your score but consider:

1: No catholic faction will usually attack papal lands.

2: The pope will very very rarely attack a catholic faction, and if so they would have been excomm already.

3: The pope will keep alliances easier with you usually until you decide to break them. Just keep building churches and priests and the occasional florin gift.

The papacy also upgrades its units over time like any other faction meaning you could gift them a powerful citadel and then they could start producing "A" units

I often use this tactic for locking the borders to Iberia,Italy,Byz,and the Mid-East.The reverse also applies if you were defending the continent, but to a lesser degree because of the steppe areas and factions you would be facing. Can also use with Denmark by taking/gifting Hamburg early to avoid war with the HRE.

I use mainly for borders, best for bottleneck provinces that HAVE to be used to get to you.