Firstly, it is clear that questions about Christianity in the past - Christian history - should be on topic. This question gives some of the reasons. However not everything that is historical, but tangential to Christianity, is on topic. Here are my suggestions:
Questions about the beliefs and practice of any Christian group, past or present, are on topic. So these would all be on-topic:
- What were the main beliefs of the Puritan movement?
- What was the Anglican church's teaching on Baptism in the 18th Century
- What did the Greek Orthodox church teach about communion?
Questions about the teachings and beliefs of specifically Christian teachers and leaders are on-topic So these would be on-topic:
- What did Erasmus believe about indulgences?
- What did Augustine believe was the proper role of the civil authorities
Questions about the beliefs of other famous people are off-topic So questions about the beliefs of these people would be off-topic:
- George Washington
- Barack Obama
Questions about the history of specifically Christian movements are on-topic. So these would all be on-topic:
- When did the Azusa Street Revival take place?
- Who was the first Anglican Archbishop of Canterbury?
Questions where Christianity is peripheral to the question, or Christian is used culturally rather than religiously, are off-topic. So these are off=topic:
- What percentage of Christians were killed by the bubonic plague?
- What weapons were used in the Huguenot (French Catholic v Protestant) Wars?
- Why did Christians infect the native American population with smallpox?
Obviously the main criterion is how closely tied the question is to the subject of Christianity. But a secondary question is where else the question might be asked. The last sections are clearly better suited to a History site, and the 'Christian Movement' questions could fit there, but I think we are more likely to be able to answer them here.
Finally, let's not forget that we don't have to be personally interested in every on-topic question. If you don't want to answer questions about historical Christianity, you don't have to. But that doesn't mean they should be banned from the site.
I'm open to suggestions about any of the areas I mentioned.