Get yourself a nice middle east territory, perhaps even an island if need be, and crank out some priests and go on a conversion war. Send 5 or 6 of em into a non Catholic territory and watch as they get traits for quickly converting the heathens =)

i've done this as far as 100+ turns in the game, and it took me about 30 turns from there to get some guys into the cardinal seats (without using assassins too)