Here is my take on the seal, and mark:
Understanding the underlying Old Testament Allusions within the book of Revelation is a key to understanding the passages' meaning. The allusions for the seal is helpful.
The mark always has its opposite, the seal of God. They key texts are Revelation 13 , Revelation 7, and 14. In each of these we learn something about the seal of God or the mark of the beast.
In the particular case of the Seal of God and the Mark of the Beast one of the key allusions is to the vision of Ezekiel in chapters 8 and 9. Here we see an account of the false worship of the Israelites, practiced by the leaders, in the temple itself. God pronounces judgement on them, and in the vision that Ezekiel sees this judgement is immediate. It foreshadows the actual judgement brought about during the destruction of Jerusalem by Babylon.
Here is the key part of the passage as it regards the Seal of God.
EZE 8:17 He said to me, "Have you seen this, son of man? Is it a trivial matter for the house of Judah to do the detestable things they are doing here? Must they also fill the land with violence and continually provoke me to anger? Look at them putting the branch to their nose! 18 Therefore I will deal with them in anger; I will not look on them with pity or spare them. Although they shout in my ears, I will not listen to them."
EZE 9:1 Then I heard him call out in a loud voice, "Bring the guards of the city here, each with a weapon in his hand." 2 And I saw six men coming from the direction of the upper gate, which faces north, each with a deadly weapon in his hand. With them was a man clothed in linen who had a writing kit at his side. They came in and stood beside the bronze altar.
EZE 9:3 Now the glory of the God of Israel went up from above the cherubim, where it had been, and moved to the threshold of the temple. Then the LORD called to the man clothed in linen who had the writing kit at his side 4 and said to him, "Go throughout the city of Jerusalem and put a mark on the foreheads of those who grieve and lament over all the detestable things that are done in it."
EZE 9:5 As I listened, he said to the others, "Follow him through the city and kill, without showing pity or compassion. 6 Slaughter old men, young men and maidens, women and children, but do not touch anyone who has the mark. Begin at my sanctuary." So they began with the elders who were in front of the temple.
As you notice the imagery of a mark in the forehead of those who are faithful to God was already seen here. And it is clear from the vision and subsequent fulfillment of the vison that
a. the mark is not a literal mark
b. The mark is a sign of loyalty to God, and grieving over apostasy and false worship.
It is not literal in the story as the true slaughter happened under the babylonians. The leaders had the opportunity to show their faith by surrendering themselves to Babylon as the prophets said, and they would not be harmed.
In revelation we likewise see God placing a sign of loyalty on those who are faithful to Him amid widespread apostasy. The remnant concept was well firmly anchored in the OT where God had a group that were faithful to Him.
Compare the above vision with this of John's in Revelation 7. Again we see the idea that those who are spared are those who receive the Seal of God.
Then I saw another angel coming up from the east, having the seal of the living God. He called out in a loud voice to the four angels who had been given power to harm the land and the sea: 3 "Do not harm the land or the sea or the trees until we put a seal on the foreheads of the servants of our God."
The mark of the beast is likewise the sign of loyalty to false worship. In fact we see in Revelation a false trinity (dragon, beast, false prophet), a false lamb (lamb-like beast), a false worship and a false seal, which is the Mark of the Beast.
The seal is being completely loyal to God in all things grieving and lamenting over all of the detestable things done in the land.
In fact, the Bible pretty much says that God's Spirit is the seal. The Holy Spirit sealed us for the day of redemption. It is a down payment guaranteeing what is to come, according to Ephesians.
Ephesians 1:13 And you also were included in Christ when you heard the word of truth, the gospel of your salvation. Having 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.
In all of the texts about the sealing the key issue was that the people were COMPLETELY dedicated to God. Just as in OT times, loyalty to God will be something that can be seen. And disloyalty to God will likewise be something obvious. What that is at this time we don't know.
The sealing is is being filled with God's Spirit, wanting what He wants and loving what He loves. Or as Revelation puts it, "following the Lamb wherever He goes."
