If the historical society is a public body and it allows other (non-religious) groups to use the space for their meetings, it cannot exclude you from the space because you are "religious."
If this is the case, go back to the historical society and point this legal fact out to them. By excluding you from public property due to the religious nature of your group, they are violating your civil rights.
I suspected as much, but we will need more legal advise like this. We are just beginning to gather our forces and will make this a legal fight. In fact, other groups do meet at the same facility. We are being excluded strictly on religious grounds. The director of the historical society indicated so over the telephone. We have asked for an explanation in writing so we can create a paper trail. Then we go to work. I'll keep y'all posted.
Actually, it's likely that the reason the study group is being asked to relocate is to avoid allowing other non-religious groups use the meeting area.