Aaaaaand, that verse has utterly nothing to do with any of that. A day late and a dollar short.
The Great Falling Away happened centuries and centuries ago, and the Man of Sin who appeared after that, and is still existing today, and will continue to exist until Christ returns, was already identified.
As for the mark, the mark is for ANGELS to identify, NOT humans, just as the opposite mark for those crying out for sin (see Ezekiel 9 after God revealed to Ezekiel the Image of Jealousy). The context in that one, was to mark which were for God and which were apostates to be destroyed, starting at His Temple. The apostates have established a counterfeit system, just as the apostates among the Jews did when they invaded the Temple and set up their carcasses and the Image of Jealousy, with the women praying to Tammuz, which resulted in their captivity to Babylon. This is the very reason the Harlot in the Revelation is called BABYLON, MYSTERY. These robbers have invaded visible Christianity, the visible body of Christians, and have established their counterfeit system with a counterfeit "christ".
Christ had warned about this long ago, saying, If one comes telling you, Here is Christ, DO NOT FOLLOW HIM.