I certainly agree that the 'Holy Roman Empire' was corrupt, downright wicked even. I also agree that the church and the state must be separate to ensure freedom to grow, develop and change our minds. The original 'pilgrims' were very well aware of religious persecution and the horrors it brings.
The fact that Christianity changed does not automatically mean that what 'we' believe today is automatically right or that we do not, to a lesser extent, fall into the same old ways of trying to impose our beliefs on others. If we have closed minds it is difficult to see the error of our ways and change/improvement is
very difficult to introduce. We should therefore, in my opinion, never be too cock-sure of ourselves on religious matters.
Someone above gave the example of abolishing slavery and introducing equality and votes for women - let me add racial equality and freedom of speech and freedom of religion! None of these changes came about easily or peaceably and we risk fooling ourselves if we assume that our individual beliefs are absolutely right and must never be changed. Each of the people who objected to those changes thought they were right. I think most of us now realize that the objectors were quite wrong. A few obviously won't.
I am very happy for everyone to believe as they wish but not for any one of us, or any group of us, to impose our beliefs on anyone else. That does not mean that we can not discuss our differences but discussion with closed minds rapidly becomes rather difficult. We should at least be able to recognize that the 200 major Christian denominations exist because we all believe different things. We can't all be right so it is highly likely that
all of us are wrong about some of our beliefs. For someone to dismiss the possibility of any error would seem like extreme arrogance to me.