We do not. We know the people who did that lived in countries where the Catholic Church was entwined with the political power structure, but that does not make them Christians. The only thing that makes a person a Christian is salvation and reformation, and what makes them identifiable by the world is living by Christ's commands. Having a government official over you who was appointed by a pope does not.
There are many things done in Christ's name that are antithetical to His teachings. That does not convey Christian status to the actors.
So, someone who says they're a Christian isn't a Christian? Hmm... interesting.
So all Christians need salvation and reformation in order to be considered a Christian. How do you prove you have salvation and reformation in order to be a Christian exactly?
Living by Christ's commands? All of them?
The 50 Commands of Christ
I've got a few websites here. This one has 50.
Do not commit adultery. So anyone who has ever committed adultery isn't a Christian.
Fear God, not man. So anyone who has ever feared man isn't a Christian.
Judge not. So anyone who has ever judged anyone isn't a Christian.
Hear God's voice. No one has ever heard God's voice, crazies think they have.
Feed my sheep. Well, I'm assuming that Christ's sheep are dead right now, unless you know of any 2,016 year old sheep out there.
So, well... interesting, Christianity just went from being the most populous religion to the least populous religion (alongside some other well know religions like the one Homer invented honoring the Fonze with ZERO participants).
Well, thanks for that.