The Resurrection
The hinge the whole faith turns on
Strip away everything else and this is the claim that made the church: that a man who was publicly executed and buried was, on the third day, alive again, not as a memory or a felt presence, but bodily, walking, eating, showing the wounds. The first Christians did not preach a beautiful teaching whose teacher had died. They preached a dead man who was no longer dead. Every other thing this faith says, about forgiveness, about hope, about God Himself, hangs on whether that one thing is true. And the remarkable part is that the Bible knows it, and says so out loud.
The faith rises or falls with it
Paul does not hedge. He lays the whole religion on a single historical event and dares it to be false: "if Christ be not risen, then is our preaching vain, and your faith is also vain" (1 Corinthians 15:14). He says it again, harder: "if Christ be not raised, your faith is vain; ye are yet in your sins" (1 Corinthians 15:17). And he refuses the comfortable exit of treating it as a nice idea regardless: "If in this life only we have hope in Christ, we are of all men most miserable" (1 Corinthians 15:19). This is a faith that made itself falsifiable. It did not say "believe anyway." It said: go check, because if the tomb was not empty, we are fools and you should walk away.
It was a body, not a feeling
The accounts go out of their way to rule out a ghost or a vision. The risen Christ ate broiled fish in front of the disciples; He invited the doubting Thomas to put a finger in the nail-marks; and He said it plainly when they thought they were seeing a spirit: "Behold my hands and my feet, that it is I myself: handle me, and see; for a spirit hath not flesh and bones, as ye see me have" (Luke 24:39). It was the same body that was crucified, the wounds still legible in it, and yet a body changed, no longer bound by locked doors or death. The church has always insisted on both halves: not a resuscitated corpse that would die again, and not a phantom, but the firstfruits of a new kind of human life, "Christ risen from the dead, and become the firstfruits of them that slept" (1 Corinthians 15:20).
The witnesses, and the awkward marks of truth
Paul hands the Corinthians a list older than any Gospel, a creed he says he "received," naming people still alive to be questioned: "he was seen of Cephas, then of the twelve: After that, he was seen of above five hundred brethren at once; of whom the greater part remain unto this present" (1 Corinthians 15:5-6). That is not the language of legend; it is the language of "ask them yourselves." And the story carries details no inventor of the time would have chosen. The first witnesses were women, whose testimony was given little legal weight in that culture, exactly the witnesses you would leave out if you were fabricating a convincing case. The tomb was empty in Jerusalem itself, a short walk from the authorities who needed only to produce a body to end the movement, and never did. And the same disciples who had run and hidden at the arrest went on to die rather than deny what they said they had seen. People will die for a lie they believe is true; it is harder to explain men dying for something they had personally staged. (See The Witnesses.)
Foretold, not improvised
The resurrection was not a happy ending invented after the fact. Jesus told His followers in advance that He would be killed and rise, and the Scriptures had pointed to it long before. David had sung what the apostles read as a promise no mere king could claim: "thou wilt not leave my soul in hell; neither wilt thou suffer thine Holy One to see corruption" (Psalm 16:10). So when Paul summarizes the gospel he twice says it happened "according to the scriptures": Christ died, and rose "the third day according to the scriptures" (1 Corinthians 15:3-4). The empty tomb was the keeping of an old appointment, not a lucky turn.
Why it changes everything
The resurrection is first of all the Father's verdict on the Son. The cross looked like God's judgment falling on a blasphemer; the resurrection reversed the verdict in public, Jesus "declared to be the Son of God with power… by the resurrection from the dead" (Romans 1:4). It is the ground of our acquittal, for He was "raised again for our justification" (Romans 4:25). And it is the death of death itself: because the firstfruits have been raised, the harvest is promised, and Paul can taunt the grave, "O death, where is thy sting? O grave, where is thy victory?" (1 Corinthians 15:55). Jesus had said as much to Martha at another grave: "I am the resurrection, and the life" (John 11:25), and to His own, "because I live, ye shall live also" (John 14:19).
This is why the resurrection is never offered as a doctrine to admire from a distance. It is a living person to meet. The same Jesus who died now speaks from the far side of death as its master: "I am he that liveth, and was dead; and, behold, I am alive for evermore" (Revelation 1:18). The cross is where He took our death; the resurrection is where He broke it. The two belong together, and neither is the whole gospel without the other (see the cross and what it accomplished).
Where this lands
If the resurrection did not happen, the kindest thing you can do with Christianity is admire it and set it down, exactly as Paul said. But if a crucified man really did walk out of his tomb, then it is the truest fact in the world, and it is not finished history but present news: He is alive, and He can be known. The creeds have said it in three plain words for sixteen centuries, "the third day he rose again" (see what the early church confessed), and the whole life of faith is simply the long answer to a tomb that would not stay shut.
Related: The Cross and What It Accomplished, What the Early Church Confessed, The Witnesses, The Trinity, and The Ascension. Scripture from the King James Version, linked to BibleHub; the words of the risen Christ are marked in purple.