I think its inquisitors jobb. I leave inquisitor and bishop in province forever. In long run it does increse zeal to 100% but its real looong run. Time to time it dropes due to inquisitor's odd jobb with fire but then it raises up more then it was before *fire*. Send 2 inquisitors in one province and it raises zeal even faster. Watch out that they don't waste your general or governor there they tend to do that sometimes even if you got a bishop there.