Most simply, people die because of the reality of sin. When Adam and Eve committed what is known as "original sin" in the garden of Eden, they ushered brokenness into the world. That brokenness includes death, as God had warned in Genesis 2:17. God said that people would return to the ground, "for you are dust, and to dust you shall return" (Genesis 3:19).
Why do people die?
Romans 6:23 tells us plainly, "the wages of sin is death." Romans 5:12 says, "Therefore, just as sin came into the world through one man, and death through sin, and so death spread to all men because all sinned." Now, we are all stained, all set for destruction and death, due to sin.
Adam and Eve did not physically die immediately, but they did experience spiritual death right away—a break in their close relationship with God. Just as we are now all subjected to physical death, we are all born spiritually dead. Our intended fellowship with God is interrupted—for each person (Ephesians 2:1). However, God has an answer for that. Due to His great desire for relationship with people, He made a way to restore that relationship.
"For God so loved the world, that he gave his only Son, that whoever believes in him should not perish but have eternal life. For God did not send his Son into the world to condemn the world, but in order that the world might be saved through him. Whoever believes in him is not condemned, but whoever does not believe is condemned already, because he has not believed in the name of the only Son of God" (John 3:16–18). The other end of the "wages of sin is death" in Romans 6:23 reads "the free gift of God is eternal life in Christ Jesus our Lord."
Jesus did away with the law of sin and death to establish the law of the Spirit (Romans 8:2). Jesus is fully human and also fully God. He lived a sinless life, died on the cross to pay the penalty for our sin, and then rose from the dead victorious. It is through putting our faith in Jesus that we have relationship with God—spiritual life. Jesus also gives us eternal existence. Those who trust in Him will still die physically, but when they do, they will be united with God in heaven (2 Corinthians 5:8). And beyond that, they will be raised to life and given bodies that are imperishable.
First Corinthians 15:20–26 sums this up: "But in fact Christ has been raised from the dead, the firstfruits of those who have fallen asleep. For as by a man came death, by a man has come also the resurrection of the dead. For as in Adam all die, so also in Christ shall all be made alive. But each in his own order: Christ the firstfruits, then at his coming those who belong to Christ. Then comes the end, when he delivers the kingdom to God the Father after destroying every rule and every authority and power. For he must reign until he has put all his enemies under his feet. The last enemy to be destroyed is death."
The breath of life – What is it?
What is the sanctity of life? Why do Christians believe in the sanctity of life?
What was the effect of the fall on humanity? How did the fall affect our world?
What is spiritual death? What does it mean to be spiritually dead?
Is there any way to guarantee ourselves a long life on earth?
Truth about Humanity