The first instance of marking in Revelation is during the first interval period, when 144,000 from Israel get sealed on their foreheads. The Greek word used in Revelation for "sealed" happens to be the same word which Paul uses when he says we are sealed with the Holy Spirit as a pledge, as the distinguishing mark of our participation in the New Covenant. It means, "to stamp with a signet or private mark." Think of a king pressing his signet ring into a blob of wax on a letter to mark it as authentic and not a forgery. So we can say that 144,000 of Israel get marked on their foreheads (and this leads to a multitude beyond counting worshiping God before His throne).
So what does that mean? The prophets shed some light on it:
"Behold, the days are coming, declares the LORD, when I will make a new covenant with the house of Israel and the house of Judah, not like the covenant that I made with their fathers on the day when I took them by the hand to bring them out of the land of Egypt, My covenant that they broke, though I was their husband, declares the LORD. But this is the covenant that I will make with the house of Israel after those days, declares the LORD: I will put My law within them, and I will write it on their hearts. And I will be their God, and they shall be My people." Jeremiah 31.31-33
"Then I will sprinkle clean water on you, and you will be clean; I will cleanse you from all your filthiness and from all your idols. Moreover, I will give you a new heart and put a new spirit within you; and I will remove the heart of stone from your flesh and give you a heart of flesh. And I will put My Spirit within you and cause you to walk in My statutes, and you will be careful to observe My ordinances." Ezekiel 36.25-27
What is new about the New Covenant is that, according to Jeremiah (and Hebrews), it is not that the word of the Lord or the commandments of the Lord have been done away with, but the new part is that now His law, statues, and ordinances will be written by God’s own finger on hearts instead of on tablets of stone.
For if He meant for His Law to be abolished with the advent of Jesus, then why say that when we receive the Spirit within us,, which was not possible until after Jesus rose from the dead and ascended to heaven, that we will walk in His statutes and be careful to observe His ordinances? What is new about the New Covenant is that now we can obey the Lord motivated by love of Him from our hearts within, instead of just by obligation or fear of punishment from without. And also, of course, the Spirit empowers us to obedience, for His fruit does not transgress His law.
So, I believe that those 144,000 who were sealed, or stamped as with a signet or private mark, were marked, as the Old Testament tells us, by the action of their hand — obedience to the word and commandments of the Lord — and the thoughts of their mind — belief in Jesus Christ, and keeping His word in their hearts and minds so as not to transgress it — through the indwelling of the Holy Spirit within them, which empowered them to walk after the example that Jesus set for us.
That brings us to the final instance of marking in the Scriptures …
To be continued …