Why the resurrection?

The answer, which most theologians missed out on, is in the story of the 153 fish after the resurrection of Jesus Christ as told in John 21:11.

When Christ shared some of the 153 fish that morning with His disciples, the action makes it a metaphor for the fulfillment of the will of the Father in Jesus Christ, who had said before He died,

My meat is to do the will of him that sent me, and to finish his work. (John 4:34)

Jesus had died for our sins and His Father had raised Him from the dead. Jesus had completed His earthly work as our High Priest by offering up His body as per the will of His Father. And the impact of that work is everlasting life for the believers.

The event that morning on the shore of Sea of Galilee is not trivial. In fact, it is the most critical of all events in the Bible because it marks the precise moment humankind can be justified before God, an event that is the culmination of a prolonged process of reconciliation between humankind and God after the fall of Adam and Eve (Romans 6:25). The resurrection of the Son is the decisive proof of the fulfillment of the will of the Father in His Son.

From an outwardly trivial action of Jesus Christ – His last meal before He ascended to heaven in His 6th and final appearance to His disciples – we reach a startling conclusion: the seemingly arbitrary number 153 is not arbitrary after all; it represents the very heart of the Christian faith.

This event, witnessed by His disciples who shared breakfast with Him that morning, has to be eternally remembered by humankind, and in the absence of a precise date, what a better way to do this than to seal the event with a numeral. The explicit number of fish, 153, is the commemoration of this momentous event.

Let me now show that indeed the number 153 in John 21:11 represents the fulfillment of the will of the Father in His Son, Jesus Christ.

Let us re-look at John 4.34. What is precisely the will of Him Who sent Jesus to live among us? The answer is in John 6:38-40:

I have come down from heaven, not to do my own will, but the will of the one who sent me. And this is the will of the one who sent me that I should not lose anything that he has given me, but should raise it to life on the last day. This is my Father’s will: That everyone who sees the Son and believes in him should have eternal life, and I will raise him to life on the last day.

In verse 44, we read how the Father chooses those He will give the Son – via His sovereignty in salvation:

No one can come to me unless the Father who sent me draws him, and I will raise him to life on the last day.

In John 17:1–2, we read that the Father gave the authority to the Son to judge all those the Father gave the Son:

After Jesus had said this, he looked up to heaven and said, Father, the hour has come. Glorify your Son, so that the Son may glorify you. For you have given him authority over all humanity so that he might give eternal life to all those you gave him.

The phrase “the hour has come” in the first verse above is a reminder of Jesus’ acceptance of the will of the Father for the Him to be crucified for our sins so that we may have eternal life. Jesus had the option to reject the will of the Father, for it is a dispositional or preferred will of the Father.

The above verses reveal a two-step process that leads to eternal life via the Father’s will for His Son. Firstly, the Father exercises His sovereignty to choose those He wants to have eternal life through His Son, and secondly, it is through the free will of those chosen to believe in His Son to have eternal life. So both God’s sovereignty in salvation and the free will of an individual determine whether the person will achieve eternal life.

With the clear understanding of the will of the Father, we re-look at John 6:39:

And this is the will of the one who sent me that I should not lose anything that he has given me, but should raise it to life on the last day.

The statement "I shall not lose anything" is an enumeration statement. Every individual given to the Son by the Father should be accounted for.

Now, let us recall the story of the 153 fish in John 21, beginning with Simon Peter urging some of the disciples to go fishing with him:

Simon Peter saith unto them, I go a fishing. They say unto him, We also go with thee. They went forth, and entered into a ship immediately; and that night they caught nothing. But when the morning was now come, Jesus stood on the shore: but the disciples knew not that it was Jesus. Then Jesus saith unto them, Children, have ye any meat? They answered him, No. And he said unto them, Cast the net on the right side of the ship, and ye shall find. They cast therefore, and now they were not able to draw it for the multitude of fishes. (John 21:3-6)

The author writes about the multitude of fishes and goes on to provide an explicit amount caught:

And the other disciples came in a little ship; (for they were not far from land, but as it were two hundred cubits,) dragging the net with fishes. As soon then as they were come to land, they saw a fire of coals there, and fish laid thereon, and bread. Jesus saith unto them, Bring of the fish which ye have now caught. Simon Peter went up, and drew the net to land full of great fishes, an hundred and fifty and three: and for all there were so many, yet was not the net broken. (John 21:8-11)

After Simon Peter pulled in the 153 fishes, Jesus invited them for breakfast:

Jesus saith unto them, Come and dine. And none of the disciples durst ask him, Who art thou? knowing that it was the Lord. Jesus then cometh, and taketh bread, and giveth them, and fish likewise. (John 21:12-13)

Jesus commanded His disciples to cast the net to the right side of the boat, for it was where all the fish had gathered, and to haul in the net with the 153 fish towards him standing on the shore. And the net did not break, for Jesus could not afford to lose any fish.

We have therefore a scenario which is a clear allegory of the Father’s will for His Son. All those chosen by the Father are gathered up and delivered to His Son to be raised on the last day and have everlasting life.

As I wrote above, the resurrection of the Son is the decisive proof of the fulfillment of the will of the Father in His Son. By John 4:34 – My meat is to do the will of him that sent me, and to finish his work – the amount of fish - Jesus’ meat - 153, therefore precisely represents that fulfillment of the will of the Father in His Son.

In summary, the number 153 in John 21:11 represents the fulfillment of the will of the Father in His Son, Jesus Christ.

Through the fulfillment of His will in His Son, the Father declared us righteous in His sight. That is, the death and resurrection of Jesus Christ is our justification, the declaration of God that we are free of guilt and penalty of sin and acceptable to Him (Romans 6:25):

He was delivered over to death for our sins and was raised to life for our justification.

Our justification also marks the beginning of sanctification, a continual process of being made holy by the power of the Holy Spirit, a lifelong process that makes us more and more like Jesus Christ.

We can argue that the Lord’s Prayer is the foremost declaration of our faith in the fulfillment of the will of the Father in His Son. Therefore, it is a means to justify ourselves to the Father. Via the Lord’s Prayer, prayed daily, we are led by the Holy Spirit in our daily lives and become more like Jesus Christ – the precise outcome of sanctification!

If Jesus had gone directly to Heaven without stopping off on Earth first, how would anyone know that he had, in fact, risen from the dead? He had to return to Earth to show people that he had risen.

Also, he had to come back in body and not just in spirit to demonstrate that we will be resurrected in the same way.

You've already received a number of great answers, but this is a central question of Christianity and the faith has developed many ways to answer it. One answer comes from the early Christian text called Hebrews. Written sometime between 50-95 A.D. by someone who was familiar with Paul's letters, the book tackles the issue of why the God of Abraham might introduce a new covenant centered on the person of Jesus. While the whole book is worth a read, perhaps the most direct answer to your question is found in chapter 7:

The former priests were many in number, because they were prevented by death from continuing in office, but [Jesus] holds his priesthood permanently, because he continues forever. Consequently, he is able to save to the uttermost those who draw near to God through him, since he always lives to make intercession for them.

For it was indeed fitting that we should have such a high priest, holy, innocent, unstained, separated from sinners, and exalted above the heavens. He has no need, like those high priests, to offer sacrifices daily, first for his own sins and then for those of the people, since he did this once for all when he offered up himself. For the law appoints men in their weakness as high priests, but the word of the oath, which came later than the law, appoints a Son who has been made perfect forever.—Hebrews 7:23-28 (ESV)

The business about "oath" versus "law" picks up on an argument begun in the previous chapter. But the main point here is that the levitical system was impermanent since:

1. high priests were subject to death, and
2. the priests themselves needed to be purified via daily sacrifice before they could offer sacrifice for the people.

It's important to note that the 1^st century Jewish concept of resurrection was not simply "going to heaven", but what N. T. Wright calls 'life after "life after death" '.¹ What Christians believe happened to Jesus was that he was born into the world with a temporary body, he was killed, and he came back with a permanent body.² God, it seems, is determined to play by His own rules and therefore required His Son to suffer death in order to obtain a resurrection body.

Jesus' death served double duty since it also functions as a final, effective sacrifice to replace the daily temple sacrifices. This aspect is developed more fully in Hebrews 10. Scholars are divided over whether Hebrews was written before or after the destruction of the Temple in Jerusalem. If it were written after, the author missed a further argument for the necessity of the sacrifice and resurrection of Jesus: the Temple sacrifices can't occur without the Temple. Without the sacrificial system, there's no levitical method for atonement of sins and cleansing of worshipers.

Conclusion

Jesus needed to be resurrected in order to obtain a permanent body so that he could be our eternal High Priest.

Footnotes:

1. I simply cannot recommend Wright's The Resurrection of the Son of God highly enough. It's a thoroughly comprehensive study of the concept of resurrection prior to and during the first century of Christianity. Even reading a sampling of sections is fascinating to any student of ancient history and philosophy.

2. Definitely not a Christian text or authoritative in any way, but I'm reminded of the fictional world of George R. R. Martin and the Drowned God ritual:

   Priest: Let your servant be born again from the sea, as you were. Bless him with salt, bless him with stone, bless him with steel.
   Response: What is dead may never die.
   Priest: What is dead may never die, but rises again, harder and stronger.

   The imaginary ritual resembles the 1^st century baptism ritual and highlights the connection to resurrection. (It also involves literal death and resuscitation, but that's a topic for a different Stack Exchange.)

For one reason, it shows that the body, as created by God is good and made for eternity.

The Manichean heresy got this point wrong and constantly pitted the good soul against the evil body. Christ's example, as being God With Us, in a bodily resurrection codifies that belief. It's a good question you raised, sometimes God teaches us truth by example and not the platitudes we expect.

The teaching of St. Pope John Paul II on the Theology of the Body is a good jumping off point if you had more questions regarding the purpose of the resurrection and the inherent goodness of the human body as well as its eternal purpose.

It also shows us a meaning for our suffering. That after we bear our cross (endure the hardships of this life), we will be given a new body, like Jesus, which will be our own, but glorified.

Christ's Resurrection has revealed "the glory of the future age" and, at the same time, has confirmed "the boast of the Cross": the glory that is hidden in the very suffering of Christ and which has been and is often mirrored in human suffering, as an expression of man's spiritual greatness. This glory must be acknowledged not only in the martyrs for the faith but in many others also who, at times, even without belief in Christ, suffer and give their lives for the truth and for a just cause. In the sufferings of all of these people the great dignity of man is strikingly confirmed.

St. John Paul II Salvifici Doloris

It's true for all of us who have bodies, not just professed Christians. The resurrection is for everyone.

This is an addendum to Narnian's answer. Jesus Christ was raised from the dead so that the people of God could stand before God blameless and holy.

• Forensically: his resurrection completed the justification of the saints (i.e. all of his people, those who he makes holy). The resurrection was God's stamp of approval on Jesus Christ, his public acceptance of his atoning sacrifice. One of the less well-known and understood verses in Paul's letter to the Romans reads,

  He was delivered over to death for our sins and was raised to life for our justification. —4:25

• Experientially: his resurrection completed the regeneration of the saints by giving them new life. As the children of God are united to the Messiah, they participate in his death and resurrection. His death destroyed the power of sin in their lives and his resurrection brought about the power of God in their lives. This is explained in detail in the middle part of Romans, particularly chapter 6.

But there's another tact that could be taken in making this answer: God does not break his promise, and he had promised that the Messiah, after undergoing suffering would reign in glory, that he would not live in Sheol, the place of shades, of ghosts. Read the Psalms; the alternatives there are not life and nonbeing, but life and Sheol. Clearly there is something deficient for a man to live only as Spirit.

• In Psalm 16:10, David in his office as the anointed of God speaks prophetically of the ultimate anointed of God: "For you will not leave my soul among the dead or allow your holy one to rot in the grave." This does not apply to David himself; he died and is long decomposed (though he awaits the resurrection on the last day). It applies to the greater Messiah, the last Messiah, the fulfilment of all God's promises, Yeshua from Nazareth.
• Isaiah 53 is abundantly clear both that the Messiah will die, but also that afterwards he will experience life and glory.

  ⁹He had done no wrong
  and had never deceived anyone.
  But he was buried like a criminal;
  he was put in a rich man’s grave.
  ¹⁰But it was the Yahweh’s good plan to crush him
  and cause him grief.
  Yet when his life is made an offering for sin [=shedding of blood/death],
  he will have many descendants.
  He will enjoy a long life,
  and the Yahweh’s good plan will prosper in his hands.
  ¹¹When he sees all that is accomplished by his anguish,
  he will be satisfied.
  And because of his experience,
  my righteous servant will make it possible
  for many to be counted righteous,
  for he will bear all their sins.
  ¹²I will give him the honors of a victorious soldier [not of a dead war hero],
  because he exposed himself to death.
  He was counted among the rebels.
  He bore the sins of many and interceded for rebels.

So the second type of answer is that Jesus was resurrected to fulfil the prophecies.

Jesus was resurrected to a physical body, and as Paul says, was the first-born with that new eternal and physical body. That is our hope now too, because of his resurrection.

Colossions 1:15-20:

The Son is the image of the invisible God, the firstborn over all creation. For in him all things were created: things in heaven and on earth, visible and invisible, whether thrones or powers or rulers or authorities; all things have been created through him and for him. He is before all things, and in him all things hold together. And he is the head of the body, the church; he is the beginning and the firstborn from among the dead, so that in everything he might have the supremacy. For God was pleased to have all his fullness dwell in him, and through him to reconcile to himself all things, whether things on earth or things in heaven, by making peace through his blood, shed on the cross.

When we die and then live in Christ, we will not be spirits, we will have physical bodies, just as Jesus did. He was resurrected so that we know that we have that same hope.

There have already been several answers here, and I don't usually post an answer to questions that have already received lots of attention, but no one here so far has touched on the central point of this doctrine: The Atonement of Jesus Christ compensates for the Fall.

Genesis gives the familiar story of how God gave to Adam and Eve a commandment and a warning: that if they broke it, they would surely die. So the Fall was the introduction of sin and death to mankind. Paul reiterates this in Romans 5: 12:

12 Wherefore, as by one man sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned.

Jesus came as our Savior, to fix what had been set wrong by the Fall. By living a life free of sin, and taking upon himself the sins of the world, he overcame sin. And by dying and being resurrected, he overcame death, and both made it possible for us to do the same. By the atoning blood of Christ, we can repent and be cleansed of our own sins. And through the Resurrection, we will be raised from death into life. Coming back to Romans 5, we read in verse 19:

19 For as by one man's disobedience many were made sinners, so by the obedience of one shall many be made righteous.

And again, in 1 Corinthians 15: 22 we see the same point.

22 For as in Adam all die, even so in Christ shall all be made alive.

This is the reason for the Resurrection: to undo death, which is an unnatural state that came upon mankind as part of the Fall, and allow us to be restored to life.

It has been well said that mankind's greatest enemy is death. Whoever would be the Savior of all mankind must then, of necessity, conquer mankind's greatest enemy. Had Jesus merely died and returned to Heaven without overcoming death, then man's greatest enemy would remain unconquered. As it is, we can ask along with the apostle Paul, "Where, O Death, is your sting?" (1 Corinthians 15:55) Death's sting has been thoroughly defeated. So, the first reason is that death had to be conquered.

Secondly, the resurrection proves that Jesus is not merely a great, albeit misunderstood, teacher, nor is He merely a prophet or a narcissist. No, the resurrection proves that Jesus is divine, as Paul states:

Paul, a servant of Christ Jesus, called to be an apostle, set apart for the gospel of God, 2 which he promised beforehand through his prophets in the holy Scriptures, 3 concerning his Son, who was descended from David according to the flesh 4 and was declared to be the Son of God in power according to the Spirit of holiness by his resurrection from the dead, Jesus Christ our Lord. Romans 1:1-4 ESV

Indeed, it was and is the resurrection of Jesus that brings credibility to His otherwise incredible claims. If there were no resurrection, Christianity would be nothing. Indeed, Paul also says that very thing:

And if Christ has not been raised, your faith is futile and you are still in your sins. 18 Then those also who have fallen asleep in Christ have perished. 19 If in Christ we have hope in this life only, we are of all people most to be pitied. 1 Corinthians 15:17-19 ESV

It is also the resurrection of Christ that gives us the hope of resurrection for ourselves.

So, three significant reasons are 1) to conquer death, mankind's greatest enemy, 2) to prove His Divinity, and 3) to give followers of Jesus the hope of resurrection.