Yeshua told His disciples not to fear death. He foresaw that they must face martyrdom at the hands of men, but He reminded them that death is not the end. There are worse fates than death. Human beings can only kill the body, but God is able to destroy both soul and body in Gehenna. Therefore we should fear God more than we fear men:
I say to you, My friends, do not be afraid of those who kill the body and after that have no more that they can do. But I will warn you whom to fear: fear the One who, after He has killed, has authority to cast into Gehenna; yes, I tell you, fear Him! (Luke 12:4-5)
In the days of the Master, the Pharisees taught that human beings possess a spirit (neshamah) that lives on after death. The Pharisees believed that this neshamah-spirit separates from the body at death. The souls of the righteous enter paradise where they wait until the resurrection when they will reunite with their bodies for the final judgment and the reward of life in the world to come. The souls of sinners enter Gehenna where they suffer punishment for sins they committed in life while they await the general resurrection when they will reunite with their bodies for judgment.
The Pharisees disagreed about whether or not the wicked would participate in the resurrection, but Yeshua and the apostles taught that God will raise all human beings to face the final judgment. “There shall certainly be a resurrection of both the righteous and the wicked” (Acts 24:15). According to apostolic theology, at the time of the final judgment, “both soul and body” of the wicked go into the fire of destruction (Revelation 20:12-15). The Master says that God will destroy both soul and body in Gehenna, and the book of Revelation says that God will cast Sheol and Gehenna themselves into oblivion:
Then death and Hades were thrown into the lake of fire. This is the second death, the lake of fire. (Revelation 20:14).
Yeshua’s disciples need not fear death in this world nor the second death so long as we remain in Yeshua. Standing on the merit and virtue of our righteous Master, we can face persecution, martyrdom, suffering, and mortality in confidence of life after death and a share in the resurrection and a share in the world to come. We have no need to fear death; death is not the end.