The first mention of the "mark of the beast" is in Revelation chapter 13.
Revelation 13:16-17 KJB
"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: 17 And 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."
The other places the mark is mentioned is in Revelation 14:9, 14:11, 15:2, 16:2, 19:20, and 20:4.
"It is utterly unnecessary to take this brand of evil literally, any more than we took the seal of Christ literally. That seal we understood as spiritual, in the faith and in the character; this evil brand we must interpret in like manner. It surely means the acquiescence in character and action to the principles of this tyrannical world-power: the right hand is the symbol of toil and social intercourse; the forehead is the symbol of character, as time is ever writing its awful tale upon men’s brows." -Charles Ellicott commentary on Revelation 13:16
"Those who understand the first beast to denote a worldly power, take the second to be also a persecuting and assumed power, which acts under the disguise of religion, and of charity to the souls of men. It is a spiritual dominion, professing to be derived from Christ, and exercised at first in a gentle manner, but soon spake like the dragon. Its speech betrayed it; for it gives forth those false doctrines and cruel decrees, which show it to belong to the dragon, and not to the Lamb. It exercised all the power of the former beast. It pursues the same design, to draw men from worshipping the true God, and to subject the souls of men to the will and control of men. The second beast has carried on its designs, by methods whereby men should be deceived to worship the former beast, in the new shape, or likeness made for it. By lying wonders, pretended miracles. And by severe censures. Also by allowing none to enjoy natural or civil rights, who will not worship that beast which is the image of the pagan beast. It is made a qualification for buying and selling, as well as for places of profit and trust, that they oblige themselves to use all their interest, power, and endeavour, to forward the dominion of the beast, which is meant by receiving his mark. To make an image to the beast, whose deadly wound was healed, would be to give form and power to his worship, or to require obedience to his commands. To worship the image of the beast, implies being subject to those things which stamp the character of the picture, and render it the image of the beast. The number of the beast is given, so as to show the infinite wisdom of God, and to exercise the wisdom of men. The number is the number of a man, computed after the usual manner among men, and it is 666. What or who is intended by this, remains a mystery." -Matthew Henry commentary on Revelation 13:16
"Applied to the papacy, the meaning is, that there would be some mark of distinction; some indelible sign; something which would designate, with entire certainty, those persons who belonged to it, and who were subject to it. It is hardly necessary to say that, in point of fact, this has eminently characterized the papacy. All possible care has been taken to designate with accuracy those who belong to that communion, and, all over the world, it is easy to distinguish those who render allegiance to the papal power." -Albert Barnes commentary on Revelation 13:16
"The mark in the right hand and forehead implies the prostration of bodily and intellectual powers to the beast's domination. "In the forehead by way of profession; in the hand with respect to work and service" [Augustine]." -Jamieson-Fausset-Brown commentary on Revelation 13:16
"No particular character is here to be understood, but only the general imposition of the Romish faith upon all sorts of persons. His mark was nothing else but either the profession of his faith and religion, or a vowed subjection to his commands, which we know is the practice of the papacy where it obtaineth in any country." -Matthew Poole commentary on Revelation 13:16
"And that no man might buy or sell - That is, this mighty power would claim jurisdiction over the traffic of the world, and endeavor to make it tributary to its own purposes. Compare Revelation 18:11-13, Revelation 18:17-19. This is represented by saying that no one might "buy or sell" except by its permission; and it is clear that where this power exists of determining who may "buy and sell," there is absolute control over the wealth of the world." -Albert Barnes commentary on Revelation 13:16
The "mark of the beast" has been being imposed on men for a long time. It is allegiance to the Catholic Church/Papacy. Don't be alarmed if perhaps you attended a Catholic church in the past, this mark is a willful mark, they that take the mark knowingly are serving this mother harlot church. They are being the willful knowing servants of evil. It would be like you now knowing what the mark is and decide you want to go participate in the Catholic church, serving and supporting it, even though it is evil. It is not a literal mark that will be imposed on men and women in the last years of the world as many would like you to think. Sadly many prophecies are being twisted by Satan's false teachers in these end times to cause fear and confusion. Do your research. Study commentary on the BibleHub website for verses you need help understanding, from trusted and proven theologians of centuries past, such as Charles Ellicott and Albert Barnes.
See also: Corrupted Catholics