Let me just explain quickly. Doors are marked by the letter nun in Arabic. The letter nun stands for Nasrani, which is “Christian”, so it's the first letter of “Christian”. Of course, we met some of these people, especially in the first refugee camp in Erbil, when we went to the Catholic church there. It brought back memories to us of the Holocaust when they used to mark Jewish people with a Star of David. It is the same concept basically. They mark your door and they give you three options. Once this mark is on your door, you have three options: get killed, pay extra tax, what is knows as jizya, or you convert to Islam. This is the meaning of the mark on the door.
On September 30th, 2014. See this statement in context.