Over the years I've been asked, "Why did Jesus have to die?" Once an earnest seeker followed the question with these comments, "As a Christian I should know the answer, but I can’t think of any convincing explanation. No leader of any other religion that I can think of ended up getting killed because of his beliefs. Why wouldn't it have been possible for Jesus to spread his message and gain adherents like Mohamed, Moses and Buddha? Does Christianity only make sense by God deciding that Jesus had to be crucified?"
This incredibly important question involves a matter central to Christianity, yet effectively answering it is often difficult for Christians. We will take a careful look at the question and lay out the answers offered in Scripture.
However, before we answer the question, "Why did Jesus have to die?" it's also important to understand that Jesus clearly understood his mission on earth, and that it involved laying down his life as a sacrifice. In other words, Jesus knew it was his Father's will for him to die.
Although God is all merciful, all powerful and all forgiving, God is also holy, righteous and just. Just consider one verse - Isaiah 5:16 - But the LORD Almighty is exalted by his justice. The holiness of God is displayed by his righteousness. (NLT).
Sin entered the world through one man's disobedience -Adam, and now all of us are born into this "sin nature." Romans 5:12 - When Adam sinned, sin entered the entire human race. Adam's sin brought death, so death spread to everyone, for everyone sinned. (NLT). And, Romans 3:23 - For all have sinned; all fall short of God's glorious standard. (NLT)
Sin separates us from God - look at what Isaiah says in 35:8 - And a highway will be there; it will be called the Way of Holiness. The unclean will not journey on it; it will be for those who walk in that Way; wicked fools will not go about on it. (NIV). Or, in 59:2 - But your iniquities have separated you from your God; your sins have hidden his face from you, so that he will not hear. (NIV)
God's holiness and justice demand that sin and rebellion be punished. The only penalty or payment for sin is eternal death. Paul states this in Romans 6:23 - For the wages of sin is death, but the free gift of God is eternal life through Christ Jesus our Lord.(NASB), and in 5:21 - So just as sin ruled over all people and brought them to death, now God's wonderful kindness rules instead, giving us right standing with God and resulting in eternal life through Jesus Christ our Lord.(NLT)
Our death is not sufficient to atone for sin because atonement requires a perfect, spotless sacrifice, offered in just the right way. Jesus, the one perfect God-man, came to offer the pure, complete and everlasting sacrifice to remove, atone, and make eternal payment for our sin. So says Peter in 1 Peter 1:18-19 -For you know that God paid a ransom to save you from the empty life you inherited from your ancestors. And the ransom he paid was not mere gold or silver. He paid for you with the precious lifeblood of Christ, the sinless, spotless Lamb of God. (NLT)Only through Jesus Christ can our sins be forgiven, thus restoring our relationship with God and removing the separation caused by sin. 2 Corinthians 5:21 - God made him who had no sin to be sin for us, so that in him we might become the righteousness of God. (NIV)The suffering and the glory of the coming Messiah was foretold in Isaiah chapters 52 and 53. God's people in the Old Testament looked forward to the Messiah that would save them from their sin. Although he did not come in the form they expected, still, their faith, which looked forward to his salvation, saved them. Our faith, which looks backward to his act of salvation, saves us. When we accept Jesus' payment for our sin, his perfect sacrifice washes away our sin and restores our right standing with God. God's mercy and grace provided a way for our salvation.
In Summary, when we are "in Christ Jesus" we are covered by his blood through his sacrificial death, our sins are paid for, and we no longer have to die an eternal death. We receive eternal life though Jesus Christ. And this is why Jesus had to die.
Material used from http://christianity.about.com/od/faqhelpdesk/a/whyjesushad2die_2.htm