Note 2 at Lu 21:18: Jesus spoke of the disciples as being betrayed by parents, brothers, relatives, and friends, and some would even be put to death (Lu 21:16). Yet the amazing statement of Christ was that not a hair of their heads would perish (Lu 21:18). The very context of this statement speaks of some of the disciples dying, so this was not a promise that no physical harm would ever come. This could only refer to the resurrection.

The Lord views everything in the light of eternity. Man tends to view things in the context of the brief life here on earth. From man's perspective, those who die for their faith in Jesus have lost a great deal. But from Jesus' viewpoint, martyrs haven't lost a thing. Even the very hairs of their heads are numbered (Mt 10:30 and Lu 12:7) and will be resurrected; plus, martyrs will receive a better resurrection (Heb 11:35).