But I also think if you have the "Lord's Mark" it cannot be taken away. If that makes sense or is Biblically correct (otherwise, let me know if that statement is wrong).
I think you are speaking of the concept of Election or Predestination found very strongly (let's say most strongly) in the book of Romans and elsewhere as well I believe. It's definitely supported by many gospel verses, particularly John's gospel springs to mind. It's a complicated topic and drove me away from the faith in college (well, to be fair, liquor and sex did that, they just borrowed the complicatedness to do it and yelled "see God's not Fair!" while they did it).
I'll put it to you this way. If He wants you - He'll have you. And if He has you at the end, He wanted you all along and once you got to know Him, you agreed. Paul knows that better than anyone, Christ appeared directly to him and flipped him 180 from enemy zealot to super-servant. Doesn't really happen unless Christ directly intervenes, reveals Himself and does that. I HATED the concept of Predestination - it was only later in life I realized it would be my only hope because I could not rescue myself from myself and I knew that what I had become did not belong to Heaven. You hate it on one side of the door and you love it on the other. Christianity is in essence, a rescue operation. (at least it was for me). And ideally, the rescue operation produces not only rescued people but also other rescuers.
As for the reference to the Lord's Mark or Seal, I think this is in different scripture. In Catholic branches you may find something similar to this upon formally joining the Church
Chrismation - OrthodoxWiki
The Christian is anointed with this oil in the sign of the Cross on his forehead, eyes, nostrils, mouth, ears, chest, hands and feet. Each time, the priest administering the sacrament says, "The Seal and Gift of the Holy Spirit."
Ephesians 1:13–14
13 And you also were included in Christ when you heard the message of truth, the gospel of your salvation. When you believed, you were marked in him with a seal, the promised Holy Spirit, 14 who is a deposit guaranteeing our inheritance until the redemption of those who are God’s possession—to the praise of his glory.
And this site has other scriptural context where Seals are referenced
What is the seal of God? | GotQuestions.org
But I also think if you have the "Lord's Mark" it cannot be taken away.
This idea that somehow the Mark of the Beast can be slipped to you surreptitiously like a Mickey in a drink is what the paranoia conspiracists play on. If it's something I see in my lifetime, I expect it will be pretty obvious. And if you have the Seal of God, it's not something you would trade it in for. And if you don't have the Seal of God... well, if you think about it...
nothing really changed now, did it? A door Christ sacrificed Himself to open for you 2,000 years ago, one that He was standing on the threshold of, just closed and locked... but... you hadn't gone through it anyway...
Kinda like showing up at one girl's house after she expressed a serious and genuine interest in you, but when you finally do, it's with another girl's lipstick freshly tattooed to your cheek. Had something like that happen in college.
SLAM!