We do not want you to be uninformed, brothers and sisters, concerning those who are asleep, so that you will not grieve like the rest, who have no hope. For if we believe that Jesus died and rose again, in the same way, through Jesus, God will bring with him those who have fallen asleep.
1 Thessalonians 4:13-14 (CSB)

These verses help us to view death in light of Jesus’s own death and resurrection. The words of Jesus and the Apostles educate believers to understand that death can be considered sleep—a lapse in consciousness and an imobilization of the body followed by renewed vigor and life. For those who are in Christ, death is transformed into sleep. As a result of this transformation, here is no reason to be overcome with hopeless mourning at the death of a brother or sister. The dead will not stay asleep forever! For that reason, Christians grieve, but not without hope or with misplaced hope.

Why can we call death sleep? Because we believe that Jesus died and rose again (v. 14). We should not press the sleep metaphor too far. Paul is not trying to give fill in all the gaps of what happens to a person's inner-person at death. His main point is that after rest, life comes. Christ conquered death and through faith we cannot be separated from him (Romans 8:38-39). Therefore, when believers die, they sleep in Jesus and await the resurrection.Mortality is gone. Death is swallowed up by life as we enter the presence of the Lord (2 Cor. 5:4). Resurrection is sure to follow. 

God will bring with him those who have fallen asleep (1 Thess. 4:14). Our sleeping brothers and sisters are not lost to history, but will return when our King comes again to reign. Those who have died—or to use Paul’s language, fallen asleep—will be raised from the dead in the same way that Jesus was raised. We will be reunited with our Christian family to live forever in fellowship with Jesus Christ at his second coming. We are to encourage one another with these words (v. 18).

Our hope provides a powerful contrast to the fear and pain of death in the rest of humanity. The world has no real answers to the tragedy of death, and many are depressed or devastated when a treasured person dies. However, we believe that those that sleep in Jesus are kept until the resurrection. It is encouraging to know that whether we are awake or asleep we may live together with him—Christ (1 Thess. 5:10). In Jesus’s resurrection we have an eternal hope that cannot be restrained by death!