Why Is the Number of the Beast 666?
G.K. Beale has a great article on The Gospel Coalition Blog about Why Is the Number of the Beast 666 in Revelation 13:18. I wholeheartedly agree with him but think that there is a layer that's missing.
Whenever you are interpreting a text, you should always looks to the closest connections first. The number "666" occurs with three people: Solomon (1 Kings 10:14 and 2 Chronicles 9:13), the returned exiles (Ezra 2:13), and the beast (Rev. 13:18).
Given how specific and rare this number is, it seems that John is alluding to one of the two of these OT references and I believe the allusion is to Solomon's 666.
1 Kings 10:14 states,
Now the weight of gold that came to Solomon in one year was 666 talents of gold
This may seem unconnected to the beast in Revelation, but when you discover the purpose and the placement of this text in the book of Kings, layers are opened up in Revelation.
Up to 1 Kings 10:14, Solomon has been a paradigmatic king: he build God's temple, he prayed for wisdom instead of wealth, and he included gentiles in both the construction of the temple and in the teaching of wisdom.
But 1 Kings 10:14 starts a sequence of three paragraphs that detail Solomon's violation of the three laws of kingship given in Deuteronomy 17:14-20: don't accumulate much gold, horses (for warfare), or many wives. Thus 666 is the pivot between the man of the highest wisdom and deepest folly; indeed deepest folly because he had the highest wisdom. Solomon, the son of David, embodies the best and the worst of humanity.
666 in Revelation
When we look at the beast in Revelation, along with those who are aligned with the beast (those who follow the beast of earth, the other beast, the harlot, etc.) commit the same three sins. As evil kings, the beast the woman and false prophet, multiply horses for warfare (Rev. 19:19), committing sexual immorality with the kings of the earth (Rev. 18:3), and make themselves wealthy on gold (Rev. 19:19).
But the point of John's identification of the beast's number as 666 is not just that they commit the same sins as a paradigmatic fallen king, Solomon, but that evil is so evil because it is the corruption of the good.
As one interpretation of Babylon holds, Babylon the harlot is Jerusalem, not (or not just) Rome. Much like Matthew's characterization of Israel as Egypt (The Art of Genius), John is communicating that the holiest city has become the harlot city, killing God's king.
This Calls for Wisdom
Before John tells us that the number of the beast is 666 he says, "This calls for wisdom: let the one who has understanding calculate the number of the beast..." Why do we need wisdom and understanding for this number?
Perhaps because the identification is so counter intuitive. If Hitler was the beast, that would not need wisdom. But if the beast were compared to a man of the highest reputation and righteous (who then fell), that would require wisdom. Nero would make sense, Solomon needs wisdom.
This is John's warning that the beast is not a foreign, nefarious creature, but us.