All the people who belong to this world (worship) worshiped "the beast" (who blasphemes God and receives his power and authority from Satan. They worship a power, an authority a creation of God but NOT The Creator - who has all power and all authority)

The number of the beast is the number of a man ( 6 - humanity). His number or what represents his name is 666. (a CREATED unholy trinity) Wisdom and understanding is needed to solve this. (7 -represents completion/perfection - God, a Holy UNCREATED Trinity)

These people (who belong to this world and worship Satan) are the ones whose... names were not written in... "The Book of Life", which (who) belongs to the Lamb (Jesus) who was killed "before the world was..... CREATED" (Revelation 13)