Rev 13:1-4
"And I saw a beast rising out of the sea, with ten horns and seven heads, with ten diadems on its horns and blasphemous names on its heads... And the whole earth marveled and followed the beast; and they worshiped the dragon, for he had given his authority to the beast."
The beast from the sea: totalitarian political power demanding worship. It combines the four beasts of Daniel 7 — a synthesis of all oppressive empires. The seven heads are seven kings/empires; the ten horns are ten future kings. Worship of the beast is worship of the dragon — political power without God is satanic power.
Rev 13:18
"This calls for wisdom: let the one who has understanding calculate the number of the beast, for it is the number of a man, and his number is 666."
666 — the most famous number in the Bible. In Hebrew gematria, 'Nero Caesar' = 666. But the meaning is deeper: 7 is the number of divine perfection; 6 is the human number (created on the sixth day). 666 is humanity exalted to the utmost without God — false perfection, usurped divinity.