Rev 13:16 And he causeth all, both small and great, rich and poor, free and bond, to receive a mark in their right hand, or in their foreheads:
From Thirteen Lectures on the Apocalypse here:
The two-horned beast "caused the earth and them who dwell therein to worship the first beast, whose deadly wound was healed." The first beast being Rome in the first stage of her Papal constitution, was now incorporate in the two-horned beast: it was merged or fused with it. This is proved by the fact that the latter "exercised all the power of the first beast"; and proved also by the history and facts of the case. The first beast was revived in the constitution of the second, in so far as imperial rank was restored to Rome, and the Roman Empire re-instated in the west, in the new empire created by the proclamation of Charlemagne as Roman Emperor by the Pope. This was the healing of the wounded sixth head, and by the public proclamation of the new empire, the earth and the dwellers therein were commanded to worship "the first beast, whose deadly wound was healed." The great wonders which the new beast was able to perform consisted of the prodigies of war, for which Charlemagne is famous in history. He was able to enforce his will upon all Europe by the powers for destruction which he possessed, symbolized by "making fire come down from heaven on the earth, in the sight of men". By these political miracles he deceived "them that dwell on the earth" into the conviction that divine authority was on his side, and proposed and carried his proposal into effect, that an image should be made of the killed beast that had recovered. This, of course, is not a literal image. It is part of the symbolism. It means a political likeness, or counterpart of the imperial system in Rome that had been killed by the Gothic sword: the restoration, in fact, of the old imperiality of Rome in a new form. This was accomplished on the investiture of the Pope with all the prerogatives of an emperor. In this position he was the exact likeness of the old Roman emperors of the sixth head -- chief magistrate in the domain of civil law, and, at the same time, chief pontiff of the national religion. He was, to all intents and purposes, an image of the defunct imperialism of the west, but a speaking image; for the new beast which came on the scene with the victories and empire of Charlemagne, had "power to give life unto the image of the beast" (verse 15). Victorious Charlemagne (accepted and crowned by the Pope) had the power to give political vitality to the Papal image of the beast. This power he exercised, and ordered the worship of the Pope-King on pain of death (verse 15), causing all to receive a mark in their right hand, or in their forehead (verse 16), that no man might buy or sell, save he that had the mark or the name of the beast, or the number of his name (verse 17). This was the symbolic way of setting forth that, under the new system, the authority of the Papal image would be made essential to the holding of any office, or the exercise of any traffic, in the emoluments or advantages of Church or State; and as this authority was conferred by signing the cross on the forehead or right hand of the recipient of official favour, the cross became the subject of this symbolism as "the mark of the beast." The name of the beast, or the number of his name, were equivalent symbols of the same thing. Those who know the name of the beast, or the number of his name, are in the secret as to what is meant by the symbol. "It is the number of a man", so says the last verse. That is, when you have found the system represented by the name of the beast, you will find the system is centred in a man, though the man and the system are two things.
I hope you find this helpful.