This is most likely an allusion back to Ezekiel 9:4-6, where the faithful believers are marked by angels, because they had fled the idolatry of the rest of people (chapter 8-9). Thus the faithful are protected from the judgment of God. Osborne writes, “A brand or tattoo on the forehead of a slave was common in the ancient world as a sign of ownership, and there were also religious tattoos to show allegiance to a particular god. Similarly, God’s slaves had his sign on their foreheads to indicate both ownership and allegiance.” While some commentators associate this “seal” with baptism, this is not the view of this author. Paul regularly refers to believers being sealed by the Holy Spirit (Eph. 1:13-14; 4:30; 2 Cor. 1:22), but there is no mention of the Holy Spirit here or baptism into Christ. Moreover, angels are never said to carry out the baptism of believers; this is done by God himself. This is, therefore, a special kind of sealing for this specific dispensation in salvation history.
Of course, after their mission is completed, it’s possible that God allows them to die. Even Dispensationalist commentator Robert Thomas writes, “After their witness is concluded, martyrdom may well be their fate.”
 Osborne, Grant. Revelation. Baker Exegetical Commentary on the New Testament. Grand Rapids, MI: Baker Academic. 2002. 310.
 Thomas, Robert L. Revelation 1-7: An Exegetical Commentary. Chicago: Moody, 1992. 475.
“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. (Rev 7:4)”