The 144,000, as mentioned in Revelation 7:1-8 and 14:1-5, refer to 144,000 Jews that were chosen by God and sealed by Him as His servants, with 12,000 coming from each of the 12 tribes of Israel. In the Left Behind books, their purpose was to evangelize to the world the gospel message about Jesus Christ being the only way of salvation. These first appeared in Left Behind and were sealed by God along with other believers in Soul Harvest. In Apollyon, they gathered at Teddy Kollek Stadium. Their task was to bring forth "soul harvest" of a Great Multitude that no one would count before the Throne of God before the Lamb as described in Revelation 7:9-10.
Notably, the book series took artistic liberty in defining who the 144,000 were, as Revelation 14:4 describes them as virginal men who "have not defiled themselves with women". Tsion Ben-Judah and Mitchell Stein are married men who became widowers. Tsion-Ben Judah mentioned that he was one of the 144,000 in Soul Harvest. Cyrus Spivey interpreted the 144,000 to be the number of righteous people who survived the Battle of Armageddon and help build the new heaven and earth.
In other religious groups
NOTE: This section contains out-of-universe material that is not part of the Left Behind story franchise.
According to the Jehovah's Witnesses, the 144,000 are those who would go to heaven to reign with Jesus, while the rest of the saved will live on an earthly paradise, similar to the world described in Kingdom Come. (Governing Body member Stephen Lett does speak of a hypothetical homosexual who was resurrected during the Millennium being killed off for failing to conform to Jehovah's moral standards after being given ample time to pursue righteousness. That person would be killed off in a hundred years, as Lett intended to refer to Isaiah 65:20, though he actually mistakenly cites verse 12.) The Jehovah's Witnesses refer to the 144,000 as "the anointed" as those who received a special heavenly calling from God's holy spirit (no capitals according to the Witnesses since the holy spirit is not person but Jehovah's active power). They interpret some verses in the Bible to apply specifically to the 144,000, such as Romans 8:14-17 about being adopted as children of God. They believe that anyone who is a baptized Jehovah's Witness could be a part of the 144,000, including married people, women, and those who are not a part of one of the twelve tribes.