14 And he deceives those who dwell on the earth because of the signs which it was given him to perform in the presence of the beast, telling those who dwell on the earth to make an image to the beast who *had the wound of the sword and has come to life.
15 And it was given to him to give breath to the image of the beast, so that the image of the beast would even speak and cause as many as do not worship the image of the beast to be killed.
16 And he causes all, the small and the great, and the rich and the poor, and the free men and the slaves, to be given a mark on their right hand or on their forehead,
17 and he provides that no one will be able to buy or to sell, except the one who has the mark, either the name of the beast or the number of his name.
18 Here is wisdom. Let him who has understanding calculate the number of the beast, for the number is that of a man; and his number is six hundred and sixty-six.
Notice the following:
1.) This number will be associated with commerce. It will hold some kind of special meaning, pledging allegiance to the Beast and his false prophet.
2.) It will be something that there is no mistaking: taking the mark will be consigning one's soul to eternal damnation irrevocably. I'm guessing it involves some kind of oath of fealty to the antichrist and his state, and expressly blasphemes the Holy Spirit, since that is expressly identified as the sole unpardonable sin in Mk. 3:28.
Anything other than that is not given by text, but human reasoning we cannot trust. I've gone as far as can be without speculation, and some of that even crept into my analysis.