All symbols in the book of Revelation are based on the Tanakh. If you know the Tanakh you know everything. Therefore 666 relates to this passage in the Tanakh,
Now the weight of gold that came to Solomon in one year was
666 talents of gold,
1 Kings 10:14
Why gold you might ask? Well, my diagram will explain it, and what the true mark of the beast is.
Rev 13:18 Here is wisdom. Let him that hath understanding count the number of the beast: for it is the number of a man; and his number is Six hundred threescore and six.
chi xi stigma
khee xee stig'-ma
The 22nd, 14th and an obsolete letter (G4742 as a cross) of the Greek alphabet (intermediate between the 5th and 6th), used as numbers; denoting respectively 600, 60 and 6; 666 as a numeral: - six hundred threescore and six.
stigma
stig'-mah
From a primary word στίζω stizō (to “stick”, that is, prick); a mark incised or punched (for recognition of ownership), that is, (figuratively) scar of service: - mark.
The mark of the beast will be something stuck under the skin used for identification of the person, and all information about them including finances, and to cut down on crime, and put in the right hand or forehead which is a position of power as they claim the man of sin as their authority.
God is at the right hand of the saints, and Jesus' name will be in their foreheads, for God is their authority.
That is why repentance and salvation is no longer available after they take the mark for they do not acknowledge any personal God, but the New Age Christ is their authority, and it is all about people with no higher power above them, so it is over and God can put down the world.
Which it will be a microchip put under the skin by hypodermic needle, and linked to satellite, and if a crime happens they will know who was in the area, and how close they were to the crime scene, and can find lost people.
The number 666 is the number of a man, the man of sin, for he is of the occult, which the occult believes in the power of numbers, and the timing, and adding, and grouping of numbers which they believe will give them more power through nature, and 6 is the number of a man, so the New Age Christ assigned the number 666, a triple 6 to himself believing he will gain more power through nature.
The beast's agenda is that there is no personal God, which he will exalt himself above all that is called God, or worshipped, and will not regard any god, but in his estate he will honor the God of forces, or the power of nature, the evolutionary process, which he believes people can still evolve to be greater and spiritual, and will say people's technology, and devices can help to achieve peace on earth.
Trump addressed this and said if the people on temporary visas had a tracking implant 911 could of been avoided.
The mark of the beast is something stuck under the skin.