In Revelation chapter 14 there is the following passage:
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 from 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 elders; and no one could learn that song except the hundred and forty-four 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.—Revelation 14:1–5
Earlier in Revelation, further insight is given into who these one hundred and forty-four thousand are:
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 servants 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: of the tribe of Judah twelve thousand were sealed; of the tribe of Reuben twelve thousand were sealed; of the tribe of Gad twelve thousand were sealed; of the tribe of Asher twelve thousand were sealed; of the tribe of Naphtali twelve thousand were sealed; of the tribe of Manasseh twelve thousand were sealed; of the tribe of Simeon twelve thousand were sealed; of the tribe of Levi twelve thousand were sealed; of the tribe of Issachar twelve thousand were sealed; of the tribe of Zebulun twelve thousand were sealed; of the tribe of Joseph twelve thousand were sealed; of the tribe of Benjamin twelve thousand were sealed.
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 voice, 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."—Revelation 7:2–17
It is still unclear what the significance of the one hundred and forty-four thousand is, but one thing is certain—they are not the only ones to come through the Tribulation and to be washed clean in the blood of the Lamb. It says that a "great multitude that no one could number of all nations, tribes, peoples, and tongues were in this multitude." It further states that this great multitude "serve [Jesus] day and night in His temple." If they serve, then they are servants. If they are servants of God, then they qualify for the seal of God wherewith the servants of God are sealed in the Tribulation.
So if you have Jesus in your heart and believe on Him as your Savior and are striving to be a servant to Him as best you can, then you will receive the seal of God in the Tribulation and will not suffer from the various plagues inflicted on the wicked in that terrible period. And if you are strong in your faith and love for the Lord, you shall "carry out great exploits and … instruct many" (Daniel 11:32–33).
But who are the one hundred and forty-four thousand? If the one hundred and forty-four thousand of Revelation chapters 7 and 14 are the same, which is a fair guess but not necessarily so, they are listed as coming from 12 of the 13 Tribes of Israel. Why the tribe of Dan is passed over is a mystery. John also said he heard the number that were sealed and then goes on to say that one hundred and forty-four thousand of the tribes of Israel were sealed. The way it is written doesn't necessarily mean that it was only the one hundred and forty-four thousand who were sealed.
The one hundred and forty-four thousand of chapter 14 are not defiled with women but are virgins. So it seems that they are men. Why women were not included is again a mystery, but this could reflect the societal standard of John's day, when women did not have the rights that they do today. Christianity had broken with that traditional view when Saint Paul stated that "there is neither male nor female; for you are all one in Christ Jesus" (Galatians 3:28).
Paul also says the marriage bed is undefiled (Hebrews 13:4), so just because a man has had sex with a woman does not make him defiled. So it seems that what is being talked about is some sort of spiritual virginity, not physical virginity. It also says they were the firstfruits, which seems to imply that they were drawn from the very early church. But then the ones in Revelation 7 seem to be sealed at the time of the Tribulation. Theories have been floated for centuries as to who the one hundred and forty-four thousand are, but the fact is we won't really know until the Lord decides to reveal who they are.