The issues of sin have already been resolved at the Judgment at the Cross. Sin isn’t the issue.
The issue is to remain in fellowship with Him in all things. There is also explicit warnings given regarding the Mark of the Beast.
There are plenty of heinous, very evil sin, much more evil than taking on a mark of the Beast or even pledging allegiance to the devil, which are forgiven upon returning to God and confessing one’s sins to Him. The significance of the Mark of the Beast is much more profound, worthy of His Prophecy some 2 millennium before the event.
Some, if not the majority of Prophecy is very literal, not symbolic.
Technically, all sin leads to death, a state of existence involving separation, but in the case of believers, our post-salvation sin isn’t as much an issue as us returning to Him, so that we might continue in His Plan, and for Him to further sanctify us.
With respect to unbelievers, they already are condemned for they do not have a regenerated human spirit, nor are able to perceive the spiritual life until they have faith alone in Christ alone.
With respect to believers, the sin unto death is a state wherein our continued life in the body and soul out of fellowship, and continuing to reject Him, no longer has potential for any good by divine standards to perform His Plan.
Many believers sin, after salvation, never able to lose salvation, but simply are leaving rewards on the shelf as eternal memorials of our foolishness.
Did you get the impression from anything I said that I thought the mark and all prophecy regarding it is insignificant? Because I acknowledge the significance of the mark and all prophecy. I am not getting your point.
I don’t agree that there is any sin more heinous than receiving the mark, which is rejecting God with the full knowledge that he exists. Revelation 14 says that those who receive the mark will experience the full power of God’s fury and wrath and will be tormented day and night forever. All those on Earth whose name is not in the Lamb’s book of life will worship the beast and receive his mark. I can see no worse sin and apparently neither can God because he plans to send them to Hell for their choice.