Rev. Mark Durie who is a human rights advocate, an Anglican priest, and an expert on Islam posted an explanation on Feb 21/2015 of the ISIS video that accompanied the slaughter of their Coptic victims. Earlier in the day he posted "Bearing the cross: a letter to the Islamic State". In that post he notes that the video is actually a letter or a message addressed to all Christians and that it was was signed with the blood of the victims.
These are not easy posts to read, but important ones for anyone who says they are Christians and thus are part of the "Nation of the Cross."
Here are the links:
Blessings in Christ