When Christ is ready to come back, and with the Holy Spirit being pour out on the faithful, a very clear choice will appear, and God's true people will step up and those who are following the guidance of the Holy Spirit into all truth will be sealed. Those who love Christ and follow what He asks will now be guided into great truths which will be made clear and much understanding unveiled, and those who truly have accepted Him will be sealed in the truth they've embraced, and those who just declared they accepted Christ but continued with their iniquity will show their true nature. Here is a good article on it:
"...What is the Seal of God?
Today, although ministers preach about the mark of the beast, relatively little attention seems to be paid to the question, "What is the seal of God? As life continues on as usual from day to day, scarcely any attention is paid to the seal of God. By contrast, nearly one hundred years ago Ellen G. White called special attention to its importance. "The time has come," she wrote, "when all who have an interest in their soul's salvation should earnestly and solemnly inquire, What is the seal of God? (Signs of the Times, Nov.1, 1899).
A century ago early believers felt that understanding the seal of God was a matter of life and death. They realized that only those receiving the seal of the living God will pass through the time of trouble and the seven last plagues successfully. Only the sealed ones will stand in the Day of the Lord. To understand this subject was a top priority.
The Seal of God as the Sabbath.
The first time that Ellen G. White associated the seal of God with the Sabbath was in 1848. A few months later, in January 1849, Joseph Bates, the pioneer Sabbath theologian, published the first Adventist book on the subject and called it, A Seal of the Living God. One of Ellen White's arguments that the seal of God is the Sabbath was that the Sabbath commandment contains the characteristics of a seal. A seal, she observed in our early days and reiterated many years later, is attached to a law to show the name, title, and authority of the lawgiver. The Sabbath commandment can therefore be considered a seal because it "is the only one of all the ten in which are found both the name and the title of the Lawgiver. It is the only one that shows by whose authority the law is given. Thus it contains the seal of God, affixed to His law as evidence of its authenticity and binding force" (Patriarchs and Prophets, p. 307).
The Sabbath helps to give the ten commandments their unique significance. "The Sabbath was placed in the decalogue as the seal of the living God, pointing out the Law-giver, and making known his right to rule." Thus the Sabbath is the sign of a relationship between God and His people, serving as "a test of their loyalty to Him" (Signs of the Times, May 3, 1886). The mission of Seventh-day Adventists can be described as "presenting the law of God as a test of character and as the seal of the living God" (Testimonies for the Church, 2:468). This reasoning seems to make good sense. However, there is more to the sealing message.
The Seal of God in the Forehead.
lf the Sabbath is the seal of the living God, what is the seal that the angel will place in the forehead (see Rev 7:2-3)? The seal to be placed in the forehead is "a mark which angels, but not human eyes, can read; for the destroying angel must see this mark of redemption. The intelligent mind has seen the sign of the cross of Calvary in the Lord's adopted sons and daughters" (7BC 968, my italics). (In this article, references to Ellen G. White comments in the Seventh-day Adventist Bible Commentary are indicated by the ab breviation BC, with the volume number preceding it.)
Described in greater detail, "The seal given in the forehead is God, New Jerusalem. will write upon him the name of My God, and the name of the city of My God, (Rev 3:2)" (Manuscript Releases 15:225).
This seal is to be given to those only who make the necessary preparation. In His mercy, God has commissioned four angels to hold the winds of strife so that His people may have time to make this preparation for receiving the seal in their foreheads (Early Writings, 38).
A distinction, evidently, was to be made between mere outward observance of the Sabbath and the true Sabbath observance which would involve intense spiritual preparation. Unfortunately, many Adventists did not take advantage of the "wind-holding" delay when they first learned about it. After more than 40 years, Ellen White exclaimed in frustration, "Brethren, how long before you will be ready for the seal of God?' (Review and Herald, June 7, 1887).
What is the Necessary Preparation?
The vast majority of Ellen G. White's comments about the seal of God deal with the preparation needed to receive it. For instance, "a great work" must be accomplished "to prepare a people to be sealed with the seal of the living God" (Selected Messages, 2:73).
A qualification found among the earliest of Ellen White's writings is that believers "must reflect the image of Jesus fully" (Early Writings, p. 7). We ask what this appealing concept means in practical terms and learn that it means to have "victory over every besetment, over pride, selfishness, love of the world, and over every wrong word and action" (ibid.). Those who "overcome the world, the flesh, and the devil, will be the favored ones who shall receive the seal of the living God" (Testimonies to Ministers, p. 445).
In order to "bear a likeness to Christ in character" (7BC 970), we are exhorted to study "more earnestly the character of our Saviour." "We should imitate the lovely Pattern that God has given us, We should dwell upon the matchless charms of Jesus until there will be nothing satisfying in this perishing world. We should desire to reflect his image in kindness, in courtesy, in gentleness, and love, then 'when he shall appear, we shall be like him; for we shall see him as he is'" (Review and Herald, May 28, 1889).
Such a focus involves the exciting prospect of becoming more and more like Jesus. "The more we study the life of Christ with a heart to learn, the more Christlike we become" (Sons and Daughters of God, p. 342). The promised outcome is beyond our highest expectations: "holiness of character" (Testimonies to Ministers, p. 446). Indeed, the cleansing of the "soul temple of every defilement," will produce characters without "one spot or stain" (Sons and Daughters of God, p. 342; Testimonies, 5:214).
Keeping God's Commandments and the Faith of Jesus.
Those who receive God's seal upon their foreheads are "God's commandment-keeping people." "Here is the patience of the saints: here are they that keep the commandments of God, and the faith of Jesus" (Rev 14:12; Testimonies, 6:15). Their sins removed, "they have on the wedding garment, and are obedient and faithful to all God's commands" (7BC 968).
To have the law of God written in heart and mind, so as to survive during the difficult times ahead, means to fill our minds with the "present truth" and love of the Bible, and to have as our greatest desire to set our eyes on Jesus at His return. "There is no place in our foreheads for the seal of the living God" if it is filled with the cares and pleasures of this world (Broadside, Jan. 3, 1849).
It is of paramount importance to manifest the works of faith. Many who teach the truth will not receive the seal of God, because even though "they had the light of truth, they knew their Master's will, they understood every point of our faith,... they had not corresponding works" (Testimonies, 5:214). "Many will not receive the seal of God because they do not keep His commandrnents or bear the fruits of righteousness" (Maranatha, p. 24 l). How serious is our behavior! "Our own course of action will determine whether we shall receive the seal of the living God or be cut down by the destroying weapons" (Testimonies, 5:212).
Keeping the Sabbath Holy.
Even though mere outward Sabbath keeping is not worthy of the seal, all who receive God's seal will nonetheless be committed to obeying the Word of God "with exactitude" (The Faith I Live By, p. 288). They carefúlly observe all ten commandments, including "the Sabbath of the fourth commandment. This is what distinguishes them from the disloyal, who have accepted a man-made institution in the place of the true Sabbath" (7BC 970, 981). The seal of God, therefore, can only be bestowed upon Sabbath- keepers.
God is very particular with His requirements. When the Israelites were about to depart from Egypt, God directed them to bring their children indoors, strike the door posts with blood, and allow no one to go outside until after midnight. When the destroying angel passed over, all who faithfully followed God's counsels enjoyed divine protection. But the first-born died in every home that neglected even the least of God's instructions (7BC 981).
The destroying angel is soon to pass agam over the world. Those who faithfully obey all God's commandments will be protected. Their unique mark will be the true, Christ-like observance of the Sabbath. Despite world-wide opposition, they choose to remain loyal, and thus they enjoy divine protection from the angel of death. They stand out among the multitude because they have a mark placed upon them, and "that mark is the keeping of His holy Sabbath." But those who are not careful in Sabbath observance will be destroyed along with the rest of the world (7BC 981).
It is no surprise that much of what Ellen G. White wrote about the Sabbath has to do with its proper observance. Believers are in danger of "doing their own pleasure on the Sabbath day" and hence are in no condition to receive the seal of the living God. "We have not, as a people, given the law of God the preeminence as we should" (Selected Messages, 3:258).
Sanctification plays a crucial role in preparation for the seal of God. There is a great need that Adventists should be "sanctified through the truth, acting upon high and holy principles, showing in a high, elevated sense, the line of demarcation between those who keep the coramandraents of God, and those who trample them under their feet." It is "the sanctification of the Spirit" that "signalizes the difference between those who have the seal of God, and those who keep a spurious rest-day" (7BC 980).
Those who fully submit themselves to the sanctifying power of the Spirit are the ones who can effectively warn the world. Their Christ-like sanctified Sabbath observance will be a crucial factor in persuading the honest in heart to accept the last message of merey. In vain have these people looked for the image of Christ in their churches (The Great Controversy, p. 390). Now, at last, they see it fully reflected in the remnant, who manifest their supreme love for the Lord of the S abbath by keeping His Sabbath holy in the finest Christlike sense.
The issue is far more than the day of worship. The day symbolizes the experience and commitment of God's true people; and even more than this, it becomes an ultimate vindication of God's character and His law, showing that what God asks is not impossible, with His power...." excerpts from 'The Seal of God' by P. Gerard Damsteegt.