A scholar argues that Nero is the person identified in the Bible as 666.
Any leader who persecuted Christians was labeled an AntiChrist but the number 6 is a reflection of the Jewish faith and comes from the 6 days of the creation and the perfect or complete number is 7 which is the day of rest, the Sabbath. When you apply it with the future Messiah, the church is in the incomplete state of 6 until He comes into the world.
The Jewish candle stand that is in the Temple has six branches connected to the center piece stem that holds the unit up as a stand. The stand center holder represents the Messiah and it is not lit until the Messiah comes in the world. When the high priest Caiphas rejected Jesus as the Messiah he became known as the first AntiChrist and maintained the number of 6. The reason St John the Apostle used 6 three times was because the Jews who rejected Jesus started persecuting the Jews who accepted Jesus. In the Jewish faith when you curse some one, you curse him 3 times, as woe woe woe, that’s like an excommunication. To him your cursing. Hence, 6 6 6 to those Jews who reject Jesus and are persecuting Jews that accepted Him. Not complicated.