With the opening of the sixth seal at the end of Revelation 6, we see the judgment of God begin. Many of the chapters between chapters 6 and 20 do not introduce new events but rather serve to amplify the events summarized in chapter 6.

Chapter 6 is an account of the events that will occur up unto the Day of The Lord, but it is a very brief summary. Chapter 7 goes on to amplify.
After these things I saw four angels standing at the four corners of the earth, holding the four winds of the earth, that the wind should not blow on the earth, on the sea, or on any tree. Then I saw another angel ascending from the east, having the seal of the living God. And he cried with a loud voice to the four angels to whom it was granted to harm the earth and the sea, saying, “Do not harm the earth, the sea, or the trees till we have sealed the servant’s of our God on their foreheads.”And I heard the number of those who were sealed. One hundred and forty-four thousand of all the tribes of the children of Israel were sealed.
The next four verses talk about 12,000 coming out of each of the twelve tribes of Israel. However, there is one tribe not listed here, the tribe of Dan. Dan is not one of the tribes which is sealed, because the people of that tribe continually engaged in idol worship and were often the first to move away from God and backslide after repentance.

The White Robed Crowd
After these things I looked, and behold, a great multitude which no one could number, of all nations, tribes, peoples, and tongues, standing before the throne and before the Lamb, clothed with white robes, with palm branches in their hands, and crying out with a loud voce, saying, Salvation belongs to our God who sits on the throne, and to the Lamb!” All the angels stood around the throne and the elders and the four living creatures, and fell on their faces before the throne and worshiped God, saying: “Amen! Blessing and glory and wisdom, thanksgiving and honor and power and might, be to our God forever and ever. Amen.”Then one of the elders answered, saying to me, “Who are these arrayed in white robes, and where did they come from?” And I said to him, ‘Sir, you know.” So he said to me, “These are the ones who come out of the great tribulation, and washed their robes and made them white in the blood of the Lamb. Therefore they are before the throne of God, and serve Him day and night in His temple. And He who sits on the throne will dwell among them. They shall neither hunger anymore nor thirst anymore; the sun shall not strike them, nor any heat; for the Lamb who is in the midst of the throne will shepherd them and lead them to living fountains of waters. And God will wipe away every tear from their eyes.” (Rev. 7:9-17)
This is an amplification of chapter 6, where John saw the souls of those who were slain for the Word of God and for the testimony which they held under the altar. They asked God how long it would be till they were avenged, and God told them to rest until all of the others would be martyred.

These in Revelation 7:14 are the same people. They are the ones who come out of the Great Tribulation. These are the Tribulation saints, or believers.

Many will be saved as a result of the Rapture. When the church goes and millions of people disappear, it is going to have a profound effect on many of the those who remain. That is why God seals 144,000 Jewish men: for the purpose of evangelism.

The result is another great harvest of souls coming into the kingdom of God. Verse 3 says, after these things. After what things? After the sealing of the 144,000, John saw a great multitude, which no one could number. A great harvest of souls will come in as a result of the work of those 144,000 Jewish evangelists.

Who They Are
Now, there are cults which teach weird things about the 144,000. There is a group called “The Manifested Sons of God” who teach that only 144,000 are going to go to heaven. How you become one of them no one really seems to know. There are others, such as the Jehovah’s Witnesses, who believe somewhat the same.

However sincere they are in their beliefs, they are sincerely wrong. The Scripture clearly contradicts what they teach. First of all, this event takes place in the hereafter. Remember, Jesus told John to write things he had seen, things that are now and things that will be hereafter.

Things John had seen were taken care of in chapter 1. The things which are were covered in the letters to the churches. Then, after the church is raptured and this age is over, we are in the things that are hereafter. That is where this entrance of the 144,000 occurs, and the church will already be gone at this point.

So if somebody is claiming he is or you can be one of the 144,000 now, he simply does not realize that the sealing of these folks will not occur until after the church is raptured.

A second reason is also clear: The 144,000 who will be sealed will be Jewish men form the tribes of Israel. Unless you or the person talking to you is a direct descendant of one of the listed tribes of Israel on the mother’s side, it would be impossible to be among the 144,000.
Then I looked, and behold, a Lamb standing on Mount Zion, and with Him one hundred and forty-four thousand, having His Father’s name written on their foreheads. And I heard a voice form heaven, like the voice of many waters, and like the voice of loud thunder.And I heard the sound of harpists playing their harps. They sang as it were a new song before the throne, before the four living creatures, and the elder; and no one could learn that song except the hundred and forty-forty thousand who were redeemed from the earth.These are the ones who were not defiled with women, for they are virgins. These are the ones who follow the Lamb wherever He goes. These were redeemed from among men, being firstfruits to God and to the Lamb. And in their mouth was found no deceit, for they are without fault before the throne of God. (Rev. 14:1-5)

Source: The Last Millennium by Mac Hammond
Excerpt permission granted by Harrison House Publishers