The number 666 is just 6 three times to affirm completeness in a human. Having the mark of the beast, the mark for destruction, is just those people looking without reservation to the teachings or
doctrines of men to save them.
[Mat 15:7-11; Col 2:20-23]

The wicked have joined a cult and are drinking the Kool-Aid of religious doctrine. Others may have the 666 as the sign of their
new name indicating
absolute faith in Christ. People who are able to buy and sell and survive life in this old world system, are those who have marked themselves one way or the other. They are using their right hand of power to conduct business and gain wealth through this old world system. Or, they are in service to God making the teachings of Christ the most important thing in their life. It thus becomes the mark in their forehead. Those who love and follow Christ are marked in their forehead to gain life.
[Eze 8:17-9:7]