Right, although not necessarily unopposed. For me, when the spirit of the devil/flesh overcomes the Holy Spirit, that is the mark of the beast - the Beast makes his mark in the life of the unbeliever, and in others' lives through his influence. When one walks in selfishness instead of godliness, in the flesh instead of the Spirit, he bears the unmistakable mark of the Beast's rule.

So the disciples had the mark of the beast when they run away the night Jesus was arrested, and Peter when he denied knowing Jesus later that night?

If they were walking in the flesh, they had the mark of the beast. And they wouldn't have been in a safe or saved condition either. To be God's child is to be led by the Spirit, not the flesh.

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We are not speaking about the "mark of the dragon" (that is, the sign of the devil's dominion over someone's life), but about the "mark of the beast." The beast is a specific agency of the dragon on earth; it's a persecuting power and its mark is its sign of authority which will be imposed by law at the time of the last great conflict of the church.

"And it causes all, both small and great, rich and poor, free and bond, to receive a mark on their right hand, or in their foreheads, even that not any might buy or sell except those having the mark, or the name of the beast, or the number of its name" (Rev. 3:16, 17).

We did switch over to a more general application, rather than the specific application found in Revelation, for both the seal of God and the mark of the beast. However, I believe that if we experience the general application of the seal of God, the specifics will be fine.

What would the specific application in Revelation be for the seal of God Arnold?

There will be two opposed groups - the 144,000 and the beast worshipers - and two opposed signs distinguishing them.
In the third angel's message, which presents the most severe warning already given against receiving the mark of the beast, we are told about what the seal of God will involve: "Here is the patience of the saints. Here are the ones who keep the commandments of God and the faith of Jesus" (Rev. 14:12). So the seal of God has to do with the commandments of God. Therefore, the mark of the beast has to do with a commandment of the beast which is in opposition to a commandment of God.

An understanding of the seal of God and the mark of the beast is one of the pillars or landmarks of Adventism. It is fundamental to Adventism. The book The Great Controversy advocates the inspired interpretation. To reject it is to reject the truth. The seventh-day Sabbath will be the focus of Satan's attack during the closing scenes of Earth's history. Any attempt to discredit this interpretation serves Satan's goals and purposes. The Bible is very upfront about it. It's not a joking matter or a trivial one. Getting it right is matter of life and death! Listen:

Revelation
14:9 And the third angel followed them, saying with a loud voice, If any man worship the beast and his image, and receive [his] mark in his forehead, or in his hand,
14:10 The same shall drink of the wine of the wrath of God, which is poured out without mixture into the cup of his indignation; and he shall be tormented with fire and brimstone in the presence of the holy angels, and in the presence of the Lamb:
14:11 And the smoke of their torment ascendeth up for ever and ever: and they have no rest day nor night, who worship the beast and his image, and whosoever receiveth the mark of his name.
14:12 Here is the patience of the saints: here [are] they that keep the commandments of God, and the faith of Jesus.

The seal of God - godliness - and the mark of the beast - selfishness - will be manifested specifically by Sabbath observance. That commandment, more than the others, is a sign of God's authority because there is no reason for it based on nature and no reason to specify the seventh day based on human logic; the reason we do it is because it is God's revealed will. We submit to God's authority, as opposed to the authority of human reason or societal pressures.

But underneath all of that, the foundation of keeping the commandments and having the faith of Jesus is the Spirit's sovereignty in the Christian. Only as we have the seal of God in that sense can we hope to pass the test of loyalty that will reveal which seal or mark we have.