It is restrictive and short-sighted to forbid questions about the history of Christianity. Like it or not, what Christians believe and do now is highly dependent on what was taught and believed in the past.
From a purely practical point of view, where are questioners going to go to get information about Christianity in the past? Are we going to need two separate sites, one for current Christianity and one for 'historical' Christianity? SE has shown many times that one site covering a wide subject works much better than many narrow sites.
Forbidding historical questions eliminates a lot of insight into current Christianity. Are we allowed to ask what John Wesley taught, or why he separated from the Anglican church? Whatever his reasons, they are relevant to Methodists today. What about the history of Bible translation? Is Augustine off-topic? Is Moody? How about Luther - he's historical, right?
And how far back does this ban go? Are we allowed to ask questions about Billy Graham crusades? They are in the past. How about the million man march? Last year's papal address?
No, questions about the history of Christianity are and should be on topic. Any other approach is going to make things much more difficult for us, and severely restrict the usefulness of the site. If you don't want to answer them, you can exclude questions with the history tag.
That is not to say that all historical questions are on-topic if they are vaguely related to Christianity. "How many Christians did the bubonic plague kill?" would be off-topic.
Finally here are a list of questions that are clearly historical but haven't been closed. Yet.