The Number of the Beast
While certainly there is more to being the Anti-Christ than bearing the number of the Beast, it is, according to most Christian tradition, an immediate indication of evil. The number "666" is the Hebrew gematria for Nero Caesar (RSKNVRN in the English equivalent to Hebrew), and it is thought that the Anti-Christ will also have a birth name which, when deciphered, equals 666. Because gematria is mystical and indefinite, there have been many discordant assertions regarding its nature, which have spawned questionable theories, such as that Adolf Hitler was the Anti-Christ, and that because Christianity is founded in Vulgar and Classical scriptures, it is the Latin gematria that should be consulted for Anti-Christian prediction.
I was freaked out when I did both my Hebrew gematria and my Latin gematria last year, and they turned out to be 665 and 667, respectively.
Here are links to websites with Hebrew and Latin gematria keys, so that you can determine whether or not you're going to be the downfall of mankind: