WHY is 666 the number of the devil?

 There is hypothesis that the number 666 came from the name of Nero. In his period, coins were stamped in the eastern part of the Roman Empire with the words in the Hebrew script of the Greek words Neron Kaisar (Emperor Nero) — "נרון קסר".  

In the event that we think about not the actual letters, but rather their mathematical qualities, their total equivalents 666. For this, a piece of a section found on the Oxyrhynchus site expresses Satan's number as 616 (in the Greek composition "χιϛ").

If we take the Nero Caesar spelling as the basis, the sum of the numerical values is reduced by 50 and equals 616, that is, Irenaeus took an erroneous spelling.