Could you be more specific about the time period you are interested in? You are talking about almost 2000 years of history.

It will vary quite a bit by time period and also by what type of power you mean. The pope has relatively limited political power now, but there are still millions that he has substantial influence with across the globe. Echoing /u/talondearg, I would suggest narrowing the time period and what type of power you're interested in.