Here is wisdom. Let him who has understanding calculate the number of the beast, for it is the number of a man: His number is 666
Revelation 13:18.
The Book of Revelation was probably written about 95 AD (I can write CE, but you know, since we’re talking about the bible here). The actual number can mean very different things. The most obvious is the Hebrew idea of gematria: letters of a name represent a number. Count them all together and you find the meaning of the name. The best known theory about the beast and the number 666 is that it’s referring to the Roman emperor Nero, the emperor that dealt with the early Christian practitioners in the years preceding the writing of the book.
The Greek version of the name for Emperor Nero is transcribed in Hebrew נרון קסר. Add it all up and you’ll find 666. A miracle. But Nero wasn’t Greek, how about that? The Latin version of his name transcribed into Hebrew is נרו קסר, which adds up to 616. Apart from that it shows the fallacy of this kind of reasoning (you can add titles, translate to any language until you find the right number), it does add up to a number that is mentioned in early versions of the manuscripts for the Book of Revelation. Not all manuscripts have the same number: some have χξϛ(666), others have χιϛ(616).
Fun fact: 666 is a so called triangular number. We can represent it by stacking, I don’t know, stuff, in a triangular shape. So we start with 6 blocks of wood. We stack 5 onto that. Then 4, then 3, 2, and finally add the top of the triangle. We now have 21 pieces. If we do that with a base of 36 blocks of wood, we get a very large pyramid and are tired in the end. Oh, yeah, and we end up with the miraculous number of the beast.
Another possible explanation about the beast is that it might mean that it is three times not complete. In this so called idealist view the number six is just shy of the divine number seven. So it is not complete. And three times that means something is three times not complete.
Be aware that this Beast is not the same as the devil. Satan is mentioned in the earlier books as the accuser, the adversary of mankind, and subordinate to God. His story is totally different.