That, my friend, is an upside-down star inscribed within a circle, otherwise known as a pentagram. Pentagrams in today's culture are often related to or associated with demonic or Satanic rituals. As for why @ItsJerry decided to watermark one over his face and demon-ify his profile picture, it was a joke, a pun, made at the fact he had 666 messages in total on the forums.
Jesus and the angels fight in a 7 year war against the Devil and his demons. Any humans that are not part of the rapture are screwed because they're left in the battlezone
For clarification: The seven years in Revalations is in reference to the Tribulation, a series of plagues similar to but much worse than those that struck Egypt in the Old Testament. Armageddon, the battle itself, will occur after the Tribulation has passed. (At least, that's how the prophecy reads.)