Six hundred threescore and six — 666 — is called the number of the beast in Revelation, the mark of opposition to God. If seven is the number of divine perfection, six repeatedly falls short: a symbol of humanity and evil straining to be god yet always failing.
Revelation calls for wisdom here, not fear. The number represents the counterfeit and the incomplete — and the whole point of the book is that the Lamb, not the beast, wins.
The Number 666 in Scripture
Here is wisdom. Let him that hath understanding count the number of the beast: for it is the number of a man; and his number is Six hundred threescore and six.