Lessons from Christian History

Although several other groups at LT have shared comments on their views of the importance (if there is any) of history, I think the topic is most important relative to the Christian Church, and what we can learn from its history.
I don't claim any awards for my scholarship or writing, but I truly believe there are important lessons for mankind in Church history. I won't start out this thread with a list of my priorities, but I pray others will add both their personal views and links to the books and writings of others. I'll be back with some of my own.

Recently I searched the web for "Beginning of Holy Roman Empire"; the responses were an appalling array of inconsistency and discrepancy. Lesson 1, perhaps, is that just because something says history doesn't assure it's true, no matter who wrote or published it. In retrospect, I suppose search engines can only show what's out there, and shouldn't be an editor. Maybe they should let readers rate the responses, or flag them, as we can rate books here.
Is Holy Roman Empire an oxymoron?

I am unable to find a separate group on Lessons of History that I referred to above, and I wonder if anyone can advise if there really is or once was such a group. I noticed a group called History Fans, which says it was refounded in the same name as a prior group which was discontinued, but it mostly comments on personal readings each month and I found no discussion on the validity of academic nor other published history. Almost daily the internet is reminding me that truth seems a dwindling proportion of public comment.