The unsaved world likes to show whose side they're on, Satan's. I humbly think the Mark of the Beast will be similar. I see women everywhere today in America covered in ugly tattoos. This is all in preparation to get people to receive the Mark of the Beast. I am not sure if the mark will be a 666 barcode, or something else.
No Christian can ever lose salvation. The Bible is clear that whoever takes the mark will be cursed. The Bible gives us a key clue in Revelation 14:11, "And the smoke of their torment ascendeth up for ever and ever: and they have no rest day nor night, who worship the beast and his image, and whosoever receiveth the mark of his name." Carefully notice that those who take the mark WORSHIP THE BEAST, as I indicated earlier with the collage of photos.
We read in Revelation 22:19, "And if any man shall take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and from the things which are written in this book." Those who corrupt the Word of God are effectively rejecting Jesus, who is THE WORD OF GOD (Revelation 19:13). I believe the same will be true of receiving the Mark of the Beast, it will effectively be a rejection of Jesus Christ.