For the Faith!
Uncommon13.7%ObjectiveOptionalSingle-player
Take part in a successful Great Holy War, on either side
You just need to be on the winning side of a Great Holy War - you don't need to lead it or be the beneficiary. Play any modest Catholic ruler in 1178, wait for the Pope to call a Crusade (he almost always does within the first twenty years), pledge on the Great Holy War screen, stack your armies with the AI's and stay pledged until it's won. Successfully defending against one as a Muslim ruler counts too - it says "on either side" and means it.