Answer
The Bible presents death as separation: physical death is the separation of the soul from the body, and spiritual death is the separation of the soul from God.
Death is the result of sin. “For the wages of sin is death” (Romans 6:23a). The whole world is subject to death because all have sinned. “Sin entered the world through one man, and death through sin, and in this way death came to all people, because all sinned” (Romans 5:12). In Genesis 2:17, the Lord warned Adam that the penalty for disobedience would be death—“You will certainly die.” When Adam disobeyed, he experienced immediate spiritual death, which caused him to hide “from the Lord God among the trees of the garden” (Genesis 3:8). Later, Adam experienced physical death (Genesis 5:5).
On the cross, Jesus also experienced physical death (Matthew 27:50). The difference is that Adam died because he was a sinner, and Jesus, who had never sinned, chose to die as a substitute for sinners (Hebrews 2:9). Jesus then showed His power over death and sin by rising from the dead on the third day (Matthew 28; Revelation 1:18). Because of Christ, death is a defeated foe. “O death, where is thy sting? O grave, where is thy victory?” (1 Corinthians 15:55, KJV; cf. Hosea 13:14).
For the unsaved, death ends the chance to accept God’s gracious offer of salvation: “People are destined to die once, and after that to face judgment” (Hebrews 9:27). For the saved, death ushers them into the presence of Christ, “to be absent from the body and to be present with the Lord” (2 Corinthians 5:8, NKJV; cf. Philippians 1:23). So real is the promise of the believer’s resurrection that the physical death of a Christian is called “sleep” (1 Corinthians 15:51; 1 Thessalonians 5:10). We look forward to that time when “there will be no more death” (Revelation 21:4).
