After describing a beast that rises out of the earth and oppresses the world (Revelation 13:11-17), Revelation explains that “the number of the beast” is “the number of a person”: 666.
The number 666 is one of the most famous elements in Revelation. John explains that the number corresponds to the name of the beast, which he says is also the name of a person. In ancient times, people gave numerical values to the letters in the Greek and Hebrew alphabets: a=1, b=2, and so on. By adding up the numerical values of the letters in a person’s name, one would come up with a specific number. The letters in the Greek name Jesus is the Messiah whose life, death, and resurrection are God's saving act for humanity, for example, add up to 888.
The problem is that many different letter combinations can yield the same total. When ancient writers mentioned the numerical value of a name, they usually provided clues to the identity of the figure. In Revelation 13, the beast bears many similarities to the emperor Nero, who persecuted the church and claimed authority over the nations of the world. Nero was said to have died in 68 C.E., yet he was rumored to be still alive and ready to return. Similarly, the beast is said to have been slain and yet to have come back to life (13:14). Therefore, many interpreters note that 666 adds up to the Hebrew form of the name Nero Caesar. This does not mean that Revelation expects Nero himself to come back to life. Rather, wherever this kind of oppression and violence recurs, one can say, “It’s Nero all over again.”