Revelation 13:18 “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.”
John tells us that the number must be “calculated,” which means there must be a hidden meaning behind it.
"The number represents Nero Caesar. The Greek Neron Kaisar adds up to 1,005, but when the name is transliterated into the Hebrew letters nrwn qsr , the sum is 666. Nero Caesar also would explain the variant number of the Beast (616) found in some New Testament manuscripts. Transliterating the Latin Nero Caesar into Hebrew, nrw qsr , yields 616, suggesting John may have been thinking of the well-known Nero Redivivus myth when writing about the Beast (i.e., that Nero would rise from the dead to destroy Christians).
The gematria solution presents three major problems: It could be viewed as cheating with the spellings (the usual Hebrew spelling for Caesar is qʾst , not qsr , although the qsr form does exist). It assumes that readers knew Hebrew well enough to do the transliteration from Greek back into Hebrew. And, perhaps most tellingly, the early Christian commentators who knew of the Nero redivivus myth never identified 666 with Nero."
Another possibility: 666 is a magic square. From very ancient times, philosophers and mathematicians were fascinated that the numbers 1–36 could be arranged in squares so that each row and diagonal would add up to the same sum (the same principle as modern Sudoku). One magic square has four rows and two diagonals that each add up to 111. The six lines of 111 equal 666. Each magic square in ancient Jewish and Greek tradition was also associated with a celestial body. In the case of the 666 square, that body was the sun, which was associated with Zeus, the highest god in the Greek pantheon.
Zeus was often associated with Baal and the mythological north ( tsaphon ) of Canaanite religion. Most relevant to the Beast of Revelation is Baal’s (and Zeus’) title, “lord of heaven” ( Baal- shamem ). It is possible that the “abomination of desolation” in [Daniel 9:27] ( shiqquts-shamem ) is a play on Baal-shamem, since the Old Testament writers sometimes substituted words that meant “shame” or “abomination” into proper names that formerly contained “Baal” (e.g., Mephibosheth, Ish-Bosheth).
This would mean 666 is a symbol for the abominable Baal, the dark lord of the Old Testament world—satanic power in New Testament thinking."—Michael Heiser