According to Danger Room things are getting bad in Mosul and the Christians are either fleeing to Iraqi Kurdistan or arming themselves:

Now, instead of relying on Baghdad to protect them, some Christian neighborhoods are taking security into their own hands, forming unofficial militias that set up roadblocks to screen for weapons and strangers. Some of the Christian militias are getting support from the Kurdish Regional Government and its security forces, according to Karadaghi.

The neighborhood militia strategy seemed to work for the Muslims, hopefully it will work for the Christians.