I delivered to you as of first importance what I also received: that Christ died for our sins in accordance with the Scriptures, that He was buried, that He was raised on the third day in accordance with the Scriptures. (1 Cor 15:3-4)So the Christian tradition is to meet and worship on Sundays because it is "the Lord's day" (Rev 1:10; see also Acts 20:7; 1 Cor 16:2) -- the day that He rose from the dead -- and we celebrate that resurrection. I suspect, however, that if you were to ask Christians, "Why is the Resurrection so important?" they might have a hard time finding much to say. So I'm here to help.
The top layer of this question is simply the "proof" nature of the miracle of His resurrection. That is, we can point to the event and say, "See? Explain that any other way than that He was who He said He was." It is the biblical nature of all miracles. Scripture refers to them as "signs", proof that the person in question is an actual emissary of God as they claim. Jesus even said, "I told you, and you do not believe. The works that I do in My Father's name bear witness about Me." (John 10:25) He said, "Do you not believe that I am in the Father and the Father is in Me? The words that I say to you I do not speak on My own authority, but the Father who dwells in Me does His works. Believe Me that I am in the Father and the Father is in Me, or else believe on account of the works themselves." (John 14:10-11) Miracles prove it. No other miracle matches His resurrection. Other resurrections occurred in Scripture, but nowhere do we find one that is predicted and accomplished someone else involved and produces a new body.
Another layer of "proof" is that the Resurrection assures us that Jesus accomplished what He intended to accomplish. If He had died and that was the end, we wouldn't have known that His sacrifice on our behalf was accepted by God (1 Cor 15:13-14). His resurrection, then, was God's indication that the price was paid and the payment was accepted. Beyond that, in a very real sense it is His resurrection that provides for our justification. Paul wrote, "It will be counted to us who believe in Him who raised from the dead Jesus our Lord, who was delivered up for our trespasses and raised for our justification." (Rom 4:24-25) The cross paid for the trespasses and the resurrection provided justification.
There are other important aspects that are often missed. For instance, without believing that He was raised from the dead, salvation is impossible (Rom 10:9). "Yeah, I believe in Jesus, but I'm not convinced that He rose from the dead" won't work.
In his speech to King Agrippa, Paul said, "I stand here testifying both to small and great, saying nothing but what the prophets and Moses said would come to pass: that the Christ must suffer and that, by being the first to rise from the dead, He would proclaim light both to our people and to the Gentiles." (Acts 26:22-23) The resurrection of Christ proclaimed light to Jews and Gentiles.
Christ's resurrection is what provides us with life as believers. We live an "exchanged life", my dead life for His living life, the "Christ in you" life (Col 1:27). That life was only made possible by His resurrection. "We were buried therefore with Him by baptism into death, in order that, just as Christ was raised from the dead by the glory of the Father, we too might walk in newness of life. For if we have been united with Him in a death like His, we shall certainly be united with Him in a resurrection like His." (Rom 6:4-5)
We are able to bear fruit as believers because He was raised from the dead (Rom 7:4).
We have a hope that there will be an ultimate resurrection for us someday. The proof of that hope is found in His own resurrection (1 Thess 4:14). Most of us have heard the biblical passage so often quoted at funerals intended to give people hope. "What is sown is perishable; what is raised is imperishable. It is sown in dishonor; it is raised in glory. It is sown in weakness; it is raised in power. It is sown a natural body; it is raised a spiritual body." (1 Cor 15:42-44) You know, "When the perishable puts on the imperishable, and the mortal puts on immortality, then shall come to pass the saying that is written: 'Death is swallowed up in victory. O death, where is your victory? O death, where is your sting?'" (1 Cor 15:54-55) That whole thing. We love that. Well, the premise of that is Christ's resurrection (1 Cor 15:20-28). How can there be any hope beyond the grave? Christ rose; so will we. Thus, we can speak with confidence, "knowing that He who raised the Lord Jesus will raise us also with Jesus and bring us with you into His presence." (2 Cor 4:14)
As we face everyday living, it is His resurrection that provides the benchmark of the power we have to live for Christ. "God raised the Lord and will also raise us up by His power." (1 Cor 6:14)
Our position, "seated with Christ in heavenly places", is proven by His resurrection (Eph 2:4-7).
His resurrection is our life (Col 2:12), our proof of forgiveness (Col 2:13), our certificate of canceled debt (Col 2:14).
His resurrection is the source of our faith and hope in God (1 Peter 1:20-21).
The Resurrection isn't a small thing. We tend to think of it as miraculous, of course, and a good thing, but it is far bigger than I think we generally see it. It is the core of the faith. It is proof -- proof that He is who He said He was, that He accomplished the payment for our sin debt and it was accepted, that there is hope, that we can be seated with Christ in heavenly places, that we are forgiven, that we can hope in God, that we will someday be with Him in heaven. His death paid for our forgiveness, but His resurrection is the reason for our justification -- that necessary exchange of our sin for His righteousness. His resurrection is the source of power for the Christian life, the reason we bear fruit as believers, the means by which we can combat sin in our own lives daily. Thus, as Paul said, it is "of first importance". Perhaps we don't appreciate it sufficiently. Perhaps we should.