144,000 Witnesses of God's Love
The 144,000 witnesses are mentioned twice in the Revelation, first in the parenthetical section between the sixth and the seventh seal judgments (Rev. 7:4-8), and second, in the interlude between the trumpet judgments and the bowl judgments (Rev. 14:1-5).
(Dictionary of Premillenial Theology, pg 424 )
Why 144,000?
The number 144,000 is the sum of twelve thousand sealed servants out of the twelve tribes of Israel, namely: Judah, Reuben, Gad, Asher, Naphtali, Manasseh, Simeon, Levi, Issachar, Zebulun, Joseph, and Benjamin. The name of Joseph stands for the tribe of Ephraim, a son to Joseph and a brother to Manasseh. The tribal name of Dan is absent. Irenaeus, a second-century church father, mentioned a tradition that the Antichrist would come from the tribe of Dan (see Gen. 49:16-18). The tribe of Dan, however, will have a presence and an inheritance in the millennial kingdom (Ezek. 48:1, 32). For some unknown reason, Dan will not contribute any to this special service during the Seventieth Week of Daniel (Dan. 9:24-27).
(Dictionary of Premillenial Theology, pg 424 )
Servants of God
The 144,000 are designated as the servants of God (7:3). The nature of their service is not given. There is some speculation that they will serve by preaching the Gospel of the kingdom on earth during the first three and a half years of the seven-year period [Tribulation] that precedes the coming of Jesus Christ to the earth (Matt. 24:14). To accomplish their service, they will be sealed by holy angels for protection from natural disasters and satanic attacks. The nature of the seal is not mentioned. It could be either visible or invisible to the human eye (Ezek. 9:4-11; Eph. 4:30). The seal on their foreheads may be seen in contrast to the mark of the Antichrist upon the foreheads of his followers.
(Dictionary of Premillenial Theology, pg 424 )
The 144,000 are Literally Jews?
The 144,000 thus comprise literal, ethnic Jews. They reveal the presence, salvation, and service of ethnic Israelites in the future Day of the Lord. Just as Jesus Christ was literally a Jew out of the tribe of Judah (Rev. 5:5), so these servants are literally Jews out of their resprective tribes. Although modern Israel does not have a system of tribal division, that system will be in place in that future day.
(Dictionary of Premillenial Theology, pg 424 )