Revelation 13:16–18 describes a future system established by the Antichrist and the false prophet, where all people must receive a mark on their hand or forehead to participate in commerce. This mark symbolizes allegiance to the Antichrist and signifies membership in his ungodly system. Without it, individuals cannot buy or sell, leading to severe persecution for those who refuse to worship the beast. The number 666, associated with the mark, represents its human, imperfect nature, falling short of God's perfect number, 777.