According to the last book in the Bible, 666 is the number, or name, of the wild beast with seven heads and ten horns that comes out of the sea. (Revelation 13:1, 17, 18) This beast is a symbol of the worldwide political system, which rules over “every tribe and people and tongue and nation.” (Revelation 13:7) The name 666 identifies the political system as a gross failure in God’s sight. How?

More than a label. Names given by God have meaning. For example, God gave the man Abram, which means “Father Is High (Exalted),” the name Abraham, which means “Father of a Crowd (Multitude),” when God promised that He would make Abraham “a father of many nations.” (Genesis 17:5, footnotes) Likewise, God named the beast 666 as a symbol of its defining attributes.

The number six implies imperfection. Often, numbers are used as symbols in the Bible. Seven typically represents completeness or perfection. Six, being one short of seven, can denote something incomplete or flawed in God’s eyes, and it can be associated with God’s enemies.—1 Chronicles 20:6; Daniel 3:1.

Three times for emphasis. The Bible sometimes stresses a matter by stating it three times. (Revelation 4:8; 8:13) So the name 666 powerfully emphasizes that God views human political systems as gross failures. They have been unable to bring lasting peace and security—things that only God’s Kingdom will achieve.

The mark of the beast

The Bible says that people receive “the mark of the wild beast” because they follow it “with admiration,” to the point of worshipping it. (Revelation 13:3, 4; 16:2) They do this by giving worshipful honor to their country, its symbols, or its military might. As The Encyclopedia of Religion states: “Nationalism has become a dominant form of religion in the modern world.” *

How is the mark of the beast placed on someone’s right hand or forehead? (Revelation 13:16) Regarding his commands to the nation of Israel, God said: “Bind them as a reminder on your hand, and they should be like a headband on your forehead.” (Deuteronomy 11:18) This meant, not that the Israelites were to mark their literal hands and foreheads, but that God’s words would guide all their actions and thoughts. Likewise, rather than being something literal such as a 666 tattoo, the mark of the beast symbolically identifies those who let the political system rule their lives. Those with the mark of the beast place themselves in opposition to God.—Revelation 14:9, 10; 19:19-21.​

More info at jw.org