Were either people aware of what the other believed in, and how similar or different were these beliefs? Could Christians living in the Arabian Peninsula at the time of the First Crusade be considered heretics by the Crusaders?
Oh I forgot to answer this question! Sorry it took so long to come back to it. But I have an answer to a similar question that should be helpful: What was the social standing of oriental christians in the crusader states of the levant?
The crusaders didn't really go into the Arabian peninsula much, and I don't know how many Christians would have been living there at the time. But there were still plenty of Christians further north, where the crusades happened.