Concurrence
It's a Christian symbol of rebellion against the tyranny of the spiritual principalities and powers that we battle in this world. This tag is used by The Voice of the Martyrs organization - to remember those that have died from persecution at the hands of the adversary.
"In northern Iraq, radical Muslims (ISIS) spray-painted the Arabic letter , or “N,” on the homes and businesses of Christians. The property owners were publicly identified as Christ-followers and given a choice to convert to Islam, leave or die. The courageous believers refused to deny their faith, and more than 100,000 fled with little more than the clothes on their backs."
Moar Sauce:
https://www.i-am-n.com