This is a bit of an exploit that I discovered while reforming my empire and banshing my vassals. Since banishment usually costs 10 piety, there'll come a time when you run out of piety. To solve this problem, create a bishopic vassals which will give you +piety (around 25+ piety, depending on the title you give him). Banish him. Rinse and repeat until you are in piety heaven. Use this to banish more people ($$$) or to get a Holy title in 3 years.