Sorry, I got messed up in semantics!
God's elect, whom I define as those described in Romans 8:28-30,
will NOT take the mark (and be lost).
But, IMO, some who are BACs WILL take the mark of the beast (and be lost).
As I told Elin, there must be 2 groups of BACs, IMO.
Further with this ...
Are the verses threatening loss of eternal life (something they already have)
written to the unsaved?
No, that does not make any sense, and it would be a waste of time!
On that idea of "wasting time", does God ever waste His time? Nope. All things work to His Plan and to His Glory. So does He give new birth to people, and then allow them to waste His time and efforts by them aborting their spiritual selves? If they are reborn, then they will not be lost, because they are no longer their own to lose. God doesn't save people to then unsaved them later. It's not something earned in the first place.
There are two categories of people who call themselves Christians (or BAC, or Believers), but God's salvation doesn't rest on how we label ourselves, but on whether or not we were elected unto Belief. "If it were possible, even the elect would be deceived", but praise God it is NOT possible. Only those fooling themselves will be fooled.
Perseverance is from the Spirit, and out of love, because He first loved us. We can't judge the heart, BUT GOD SURELY WILL! We can fool those around us into believing that we have Faith, and we can even fool ourselves into thinking we have Faith, BUT YOU CAN'T FOOL GOD. He knows His Sheep, and keeps His Sheep, and will not lose a single one.
If we have Faith, we will express Faith through faithfulness, resulting in Good works and the fruit of the Spirit.
Yes, we must endure, and if we are in Christ, then we WILL endure. No one who has been adopted shall be disowned, and no Bride will be divorced... either He knows you... or He NEVER knew you. There is no "BAC" who will be forgotten about on the Last Day... only those who pretended to be "BAC" that will have a rude realization of where their belief was truly placed, and who they had actually been serving.