I John 5:16-18
If anyone sees his brother commit a sin that does not lead to death, he should pray and God will give him life. I refer to those whose sin does not lead to death. There is a sin that leads to death. I am not saying that he should pray about that. All wrongdoing is sin, and there is sin that does not lead to death. We know that anyone born of God does not continue to sin; the one who was born of God keeps him safe, and the evil one cannot harm him.
I'm not sure I agree with the parallel argument you suggest. I'll have to think about that one and get back to you.
My point in the statement you quoted was really that loving and serving God is a choice and it is, by definition, the opposite choice of rejecting God. Those receive the mark will make a conscious choice to reject God. We have already made our choice and therefore need not fear changing our minds. The parallel you offer is that of choosing sin. Continuing to sin as a believer is the flesh struggling with the spirit. There is no parallel struggling that takes place in loving and serving God. You either love him or you don't. (Note that I'm not implying that there aren't times when we are closer to God in our walk with than at other times.)
I'm not following your last two paragraphs. To what are you referring?
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.