Looking at the data files the general theory is that for the "Faith" traits, the % of your religion needs to change by a large amount (8%) and your priests must have recently arrived in the province (<3 turns). For the higher levels it is 10% and 5 turns. You get Missionary and Heresy bonuses from recently converted provinces that have >95% your religion, i can't make sense of the details. Anyway the common strategy is the "God Squad", build as many priests as you can (at least 5) and send them in one big group along with your cardinal(s) to lands with low (0-30%) Catholic. Concentrate on one region at a time until it is >95% before moving on to the next. Your priests will gain piety very quickly, especially if there are a few heretics for them to denounce. Your cardinal(s) will also gain piety, and move to the top of the ballot for pope.