Let he who hath understanding reckon the number of the beast for it is a human number.
It's number is six hundred and sixty six.
-Revelations chapter 13, Verse 18. As said in "The number of the beast" by Iron Maiden
"...And that number is 666."
Widely implied that it was 666 because it was an imperfect number to represent the impurities of man and their evildoings and sinful ways.
Read the book of revelations, at the end of the New Testament; it tells all about that, the seven seals, the apocalypse, Armageddon, Megiddo, etc...