The death of a believer is the death of his mortal body, but not the end or defeat of his life in Christ: at death he remains with Christ, and his body awaits the promised resurrection.
For the Christian, to die is gain, and to be absent from the body is to be present with the Lord. Death remains an enemy, but a defeated one: it can end the mortal body, yet cannot sever the believer from Christ, whose risen life he shares. What death takes is given back and more at the resurrection, when this corruptible shall put on incorruption.
See in Scripture: Luke 23:43; Romans 8:38–39; 2 Corinthians 5:6–8; Philippians 1:21–23; 1 Thessalonians 4:13–14