In Revelation 7, verse 1, we find that the Day of the Lord—a time of terrible plagues—is to be delayed temporarily, until the servants of God have been sealed (verse 3). According to the sequence of prophesied events, this sealing is yet in the future. It will occur after the coming Great Tribulation and the appearance of the heavenly signs, but just before the plagues of God's judgments are poured out.
Next, notice Jesus' own explanation of the 144,000: They are of the tribes of Israel, 12,000 of each tribe (verse 4), who are set apart from the rest of humanity (see Ezekiel 9:3-10 for a similar occurrence). In Galatians 6:16, the church is called "the Israel of God," so this separation into "tribes of the children of Israel" may be a spiritual designation rather than physical lineage.
The 144,000 are sealed by the Holy Spirit in their foreheads (minds). Revelation 14:1 shows that this sealing is done by writing the Father's name there. Jesus' very last prayer was that His church would be kept—preserved, guarded—in the Father's name (John 17:11). This suggests that the 144,000 are set apart, and thus protected (in an eternal sense), by God's work of building character in them, as God's name identifies and defines His character.
So, like those in Ezekiel's day who "sigh and cry over the abominations that are done," the 144,000 will be spared the terrors of the Day of the Lord because they have proven their sterling character before God. This is confirmed in Revelation 14:4, which describes them as "not defiled with women, for they are virgins," that is, morally upright and undefiled by false religion. Verse 5 adds, "And in their mouth was found no guile, for they are without fault before the throne of God." Their sealing is obviously based on their spiritual condition rather than their physical descent.
They are also called "redeemed from among men, . . . firstfruits to God and the Lamb" (verse 4). This can only mean that they are those who take part in the first resurrection (Revelation 20:4-6; I Corinthians 15:23). They are a special first "crop" of God's harvest, who are given special rewards and privileges because of their struggles in overcoming of Satan and their sins—Hebrews 11:35 calls it "a better resurrection."
The 144,000 are a unique group of people who overcome their sins and grow in character, and their reward is to live and rule forever by the side of Jesus Christ.