King Louis II of Italy in 867 offers the best start both because of the level of devotion and because Catholicism has the slowest Fervor growth. Then get the skill Prophet in the learning tree which lets you create a faith cheaper. Now wait until you get enough religion points and than create the faith in the religion menu.
Credit to Bayek von Kuwaq on Steam for his guide
https://steamcommunity.com/sharedfiles/filedetails/?id=24324...