Here is a cut from my website on the subject:
7:1 And after these things I saw four angels standing on the four corners of the earth, holding the four winds of the earth, that the wind should not blow on the earth, nor on the sea, nor on any tree.
7:2 And 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 given to hurt the earth and the sea,
7:3 Saying, Hurt not the earth, neither the sea, nor the trees, till we have sealed the servants of our God in their foreheads.
7:4 And I heard the number of them which were sealed: and there were sealed an hundred and forty and four thousand of all the tribes of the children of Israel.
14:1 And I looked, and, lo, a Lamb stood on the mount Sion, and with him an hundred forty and four thousand, having his Father's name written in their foreheads.
These 144,000 sealed ethnic Israelites are a unique group composed of 12,000 from each of 12 tribes of the children of Israel. They are ethnic Israelites of the nation.
Many theories have been presented describing this group. Some Gentile Christian denominations have identified themselves as the 144,000 and there are those who advocate a general application which includes all believers.
Neither of these interpretations is correct. The 144,000 are 12,000 from each of the 12 tribes of the children of national Israel, they are believers, and they will be present during the coming tribulation period .... the Lord's hour of trial against an unbelieving world. [Revelation 7:1-8; 14:1-7]
These will be the first converts [fruits] of the Lord's during the coming tribulation period of His wrath. The link to their Israelite ancestry is given describing the practice of idolatry that prevailed where the kings of Israel [like Solomon] were lead to worship foreign gods by women .... these 144,000 will not follow this course during the tribulation period, but will be loyal to the Lord.
Their mission will begin in Israel at the beginning of the tribulation period [the 70th week decreed for the nation] and will reach other Middle East nations and beyond.
By the middle of the week they are seen in Jerusalem with the Lord [He will be present, but not seen by the world until the end of the period] and they will take the gospel to the whole world during the tribulation period. [Revelation 14:6-7]
There are a few direct explanations about this group, their purpose, and their activities appearing in the Book of Revelation, but no where else in scripture. These servants of the Lord are sealed with His protection.
Their sealing will include protection against the attacks by the angels of the abyss during the 5th trumpet judgment and all other dangers during the 70th week including the Lord's judgments and those instigated by satan and his agents. [Revelation 9:4]
They will also be the first fruits [first believers] of the tribulation period and will enter the Lord's millennial kingdom as surviving mortals. [Matthew 24:29-31; Revelation 14]
The 144,000 Israelites are not to be confused with the church [the 24 elders and the great multitude] which is composed of both Israelite and Gentile believers [Old and New Testament periods]. These will dwell in heaven during the Lord's coming time of judgment upon the earth [Revelation 4:4; 5:5-10; 7:9-17; 11:1; 12:12; 19:1-14; and 20:4 (those on thrones)]
Both of these groups will be protected from the judgments of the tribulation as the result of the Lord's immortalization of the church and the sealing of the 144,000 mortal Israelites on the earth. The Lord will enter the material world unexpectedly as a thief and execute both of these events. [1Corinthians 15; 1Thessalonians 4; Matthew 25:1-13; Revelation 3:10; 18:4; 7:1]
The 144,000 sealed Israelites will be set apart from all other unsealed mortals who will experience the tribulation. They will be the only living believers [first fruits of redemption] on earth at the beginning of the 70th week just after the church is taken by the Lord to heaven.
There will be other unsealed humans who will become believers during the tribulation and, but not many [most humans will not repent]. The Lord will resurrect those believers who die physically by beheading [martyred] at the end of the tribulation period. [Revelation 20:4 (those beheaded)]
All believers who remain alive [survive the tribulation] will enter His earthly kingdom as mortals. He will gather and sort out these believers from all of the humans left in the nations of the world just after the coming tribulation period. [Matthew 24:29-31; 25:31-46]
The "elect" will consist of believing Israelites [His brethren] and the sheep will be the believing Gentiles. These will all be mortals who will enter and populate His worldwide millennial kingdom which will last for 1,000 years.
The rest of the survivors will be unbelieving [goats] who will not enter the kingdom, but will be cast off into the outer darkness.
All unbelievers will be resurrected [the second type of resurrection to the second death] to face judgment and elimination. Their names will not be found in the Lord's book of life. [Revelation 20:6-15]
This is the resurrection that leads to the second death. The first resurrection type is the resurrection to life. The Lord was the first to experience this type of resurrection, the church will be next at His second advent to immortalize the believer, and the tribulation martyrs will be resurrected at the end of the period. [1Corinthians 15:22-58; Revelation 20:4 (those beheaded)]
There will also be a believing Israelite remnant who will be living in the land of Israel during Israel's defeat at the hands of the little horn [satan's beast].
It is these who will flee into the Jordanian wilderness when they see the abomination of desolation standing in the holy place [temple mount].
Their escape route will be along a great rift valley that is formed by the Lords who will split the Mount of Olives to the north and to the south. This event is recorded in Zechariah 14, Matthew 24:15-16, and Revelation 12.
These will be kept from the reaches of Abaddon-Apollyon [the beast in the little horn] whose armies will occupy and rule the nation of Israel for 42 months [1,260 days]. [Revelation 11:2; 13:5]
This Israelite remnant will receive the Lord as their Messiah during the tribulation period and will enter and populate His coming kingdom on the earth as mortals at the end of the end of it.
This remnant is not to be confused with the sealed 144,000 Israelites who will spread the gospel of the kingdom. The 144,000 will be active during the entire tribulation period. First to Israel, and then to the entire world. [Matthew 24:14; Revelation 14:1-7]
A more comprehensive understanding of the purpose of the 144,000 Israelites can be developed by researching Revelation for the Lord's continuous provision of individual salvation during the tribulation.
The coming tribulation period [the Lord's hour of trial] experienced by Israel and the nations will be a very confusing and perplexing time with regard to the events that will be taking place which will be very difficult for humans to understand and experience.
The Lord will provide a very strong measure of the truth about what is happening in proportion to the confusion, deception, and the judgments during the period.
The first seal that He opens is the symbol of the white horse which depicts one of six themes [seals] that will be characteristic of the conditions of the tribulation period. [Revelation 6:1-2]
The symbol of the white horse represents an ongoing communication of the truth about the Lord, personal salvation, and His soon coming 1,000 year earthly kingdom which will emerge in the wake of the destruction of human self rule.
This activity will begin at the onset of the Lord's hour of trial and will continue throughout the tribulation period. The white horse is the symbol of the Lord's conquering salvation.
The 144,000 will be a powerful force that the Lord will use to continue spreading the gospel of the kingdom and of individual salvation to an unbelieving world during the tribulation period.
They will also serve as an inspiration for those believers who will be present during the tribulation period. Many of these will be martyred for their faith. [Revelation 6:9; 14:13; 15:2; 20:4 (those beheaded)]
An angel will appear having the everlasting gospel that will be preached to all that live on the earth during the coming tribulation period. This angel will appear at the beginning of the period and the 144,000 Israelites will begin their mission in Israel and then will reach the whole world. [Matthew 24:14; Revelation 7; 14]
The message will be to fear and worship the Lord, that His judgment is in process, and that His visible appearance to establish His Kingdom is at hand. He said that this gospel of the kingdom will be preached to the whole world just before His visible return.
The 144,000 are seen by John on Mount Zion [in Jerusalem] with the Lord [the world will not see Him until just after the tribulation period is over]. Their sealing will take place just before the beginning of the tribulation period.
The Lord will be present during the entire period of His hour of trial [He will not be seen by he world during this time frame] and He will lead the 144,000. He opens the seals, controls all of the judgment events of the period, and His visible appearance to the world is scheduled immediately after the tribulation period.
The Lord can have an invisible presence or a visible presence in relation to His involvement with the material world. Scripture records events and evidence of this unique ability at His first coming. He will function in the same manner during His involvement at the time of the end.
He can be present and invisible to humans during one encounter, and visible and present in another. He will be present and invisible during the tribulation period, and then visible to the world when He appears at the end of it.
Heaven is not a place in some far off corner of outer space .... it is a state of existence which allows for optional entry or exit of the material universe.
There is a great deal of confusion concerning the event of the Lord returning from heaven visibly at the end of the tribulation period. Some interpret this as nullifying the possibility of His invisible presence for involvement in the events of His hour of trial that will lead up to His visible appearance at the end of it.
His first action will be His unannounced and sudden invisible presence as a "thief" to immortalize dead and living believers of today's church. This action will take place just before the day of the Lord begins .... His judgment of an unbelieving world, and beyond.
The day of The Lord is not just one 24 hour day when He appears visibly to the world. His "day" will include bringing man's day [human rule] to a close at the end of this present age first .... and then He will establish millennial kingdom on the earth.
The primary mission of the 144,000 Israelites will be to communicate the message of personal salvation and the Lord's soon coming earthly kingdom.