"Also it causes all, both small and great, both rich and poor, both free and slave, to be marked on the right hand or the forehead, so that no one can buy or sell unless he has the mark, that is, the name of the beast or the number of its name."
--- Revelation 13:16-17
"If anyone worships the beast and its image and receives a mark on his forehead or on his hand, he also will drink the wine of God's wrath, poured full strength into the cup of his anger, and he will be tormented with fire and sulfur in the presence of the holy angels and in the presence of the Lamb."
--- Revelation 14:9-10
It is unclear from scripture what exactly the mark of the beast is.
It is my belief that the mark will change our genetics, making us "unsavable." This parallels what happened in the days of Noah when demonic angels contaminated the gene pool.
Some believe it will be a chip implant. The Bible does not say.
What we do know for certain from the Bible is that if you take the mark of the beast, you CANNOT BE SAVED. There will be no repentance available after taking the mark.
Taking the mark of the beast is "blasphemy against the Holy Spirit," which the Bible calls an unforgivable sin.
(Matthew 12:31 NLT, Revelation 14:11)
Sadly, there will probably be some "born again" Christians who take the mark of the beast. These are true Christians, not pretend ones.
I believe that scripture possibly gives us a foreshadowing of this in the Garden of Gethsemane. One of the disciples, Judas, betrays Jesus in the garden. Judas turned from following Jesus (John 6:66) and betrayed Him.
Could this foretell that about 1/12, or 8%, of true Christians will take the mark of the beast?
If so, it is sobering.