“The number of the beast... 666”
so that no one could buy or sell unless he had the mark, the name of the beast or the number of its name. Here is a call for wisdom: Let the one who has insight calculate the number of the beast, for it is the number of a man, and that number is 666. (Revelation 13:16–18 BSB)
What it sounds like today
Two millennia of speculation, most of it ignoring the sentence's own instructions.
The cultural key
'Calculate' invokes gematria, the letters-as-numbers practice this site met at Matthew's fourteen generations. The oldest and strongest solution: Neron Qesar (Nero Caesar) spelled in Hebrew letters sums to 666, and the clincher is textual: some early manuscripts read 616 instead, which is exactly what you get when Nero's name is spelled from Latin without the final N. Two number readings, one man, each matching a spelling. The 'mark' (charagma) was the word for the imperial stamp on documents and coinage, and marking hand and forehead parodies the tefillin worn at exactly those points (Deuteronomy 6:8): allegiance worn on the body. 'Cannot buy or sell' touches the guild-and-market life where cult participation was the price of commerce (the idol-food problem's harder sibling).
Recovered meaning
For the first hearers: the beast has a name you already know, the emperor-cult's economy already presses you, and the mark is a counter-tefillin, worship stamped where God's word belongs. The number was a coded confirmation to insiders, not a puzzle aimed at distant centuries, though the pattern it names keeps finding new candidates.