conversion is the fastest way. I alway crank out 4-5 priests for each 0 catholics region i can easily send them to. once it reaches 50% or so, i move on. It is the way the conversion is calculated.

You convert x% of NON-catholics. if x=10 and there is 100 population, you can convert 10 on the first turn. On the second turn, you only convert 10% of 90 (since 10 are already converted), i.e 9 people only and so on...so you convert less and less people if you stay in the same region for too long. With less people converted, your priest rank up slower.