The Mark of the Beast (which is the number 666) is a symbol of loyalty to the Beast out of the Sea as prophesied in the Book of Revelation. Anyone who refuses to take this mark will be cut off from society. This is a deliberate imitation by the ungodly beast system to the work of God with those symbolic 144,000 sealed believers earlier in the Revelation.
John appeals to the readers of Revelation, noting that understanding of the number requires wisdom and can only be calculated by the few.
The Beast out of the Earth will make an image seem to be alive. That image will order everyone to receive a mark on the right hand or forehead as a symbol of their loyalty to the first Beast. Anyone who who refuses to take the the mark will be unable to legally buy or sell, and be subject to arrest and execution.
The mark will be associated with the name of the beast by virtue of the nature of numerals in the ancient world. The languages in which the Bible were written had no separate symbols for numerals, which were usually spelled out. Consequently, to save space, letters had values, making words have a value as well. Sometimes letters laid beside each other might actually spell a word and be associated with it. In the case of this Greek alpha-numeric symbol, most texts have χξς, using an obsolete letter as the six. The full number is 666, called the "number of (a) man", the beast.
As a pictogram, the visual of the three symbols suggests a cross, a snake and a mark made by an injury. In ancient times this was made as an "X" but represent a different letter from the Greek χ (chi). The last letter, letter called a "stigma" with the sound of "st" which means "mark". However, the original "6" was the letter Ϝ , called the digamma, or more correctly the waw, reflecting the semitic (and Hebrew) letter with the same value and meaning "nail". The middle letter, though it makes the /ks/ sound in Greek just "happens" to have the same value as the Semitic samekh which has the /s/ sound. The overall picture remains the same, but the enigmatic "666" changes.
This leave open an intriguing mystery which may have been lost in translation from a possible Aramaic original of 𐡎𐡅χ tau-samekh-wau. All the looking for a villian, like Nero, may be pointing in the opposite direction. The Revelation is a very Jewish vision, and the Jewish believers in the churches may have been the ones with the key to who the number was referring. The value of the number changes to 466. The significance of this possibility demonstrates that it will, or did, take true wisdom to discern the meaning of the number.
The Significance of the Mark
The practice of marking a loyal servant, or a slave, goes back to ancient times. In the Law, the servant who was due to be released on the Sabbath year could declare his loyalty by having his ear pierced, or notched, permanently marking him. Later, in a vision, Ezekiel is shown an man in a white robe marking righteous people with ink on their foreheads, a practice reflected in the sealing of the 144,000. Those who take the mark are usually willing recipients, or trusted confidants marked for service or protection. If the mark is applied in chattel slavery, no loyalty is assumed. The mark of the beast is not one of possession, but of loyalty.
Having accepted the security granted by the government of the beast, bearers of his mark, "earth dwellers", will find that they ultimately fall under the wrath of God. In the bowl judgments they are specifically said to suffer greatly in pain and darkness.