It is commonly assumed that the gospel message which Jesus Christ brought is that He died for our sins. Many believe I Corinthians 15:1-3 backs this up. However, focusing on this passage to the exclusion of what Jesus Himself said, as well as the context and the rest of the chapter, gives an incomplete picture. I Corinthians 15:1-3 says,
Moreover, brethren, I declare to you the gospel which I preached to you, which also you received and in which you stand, by which also you are saved, if you hold fast that word which I preached to you—unless you believed in vain. For I delivered to you first of all that which I also received: that Christ died for our sins according to the Scriptures. . . .
While Jesus Christ's death for our sins is, without question, extremely "good news," it is in fact only a part of the reason that He came to earth, as the rest of the context shows. But before looking at the rest of I Corinthians 15, consider what message Jesus Christ brought:
And Jesus went about all Galilee, teaching in their synagogues, preaching the gospel of the kingdom, and healing all kinds of sickness and all kinds of disease among the people. (Matthew 4:23)
And Jesus went about all the cities and villages, teaching in their synagogues, preaching the gospel of the kingdom, and healing every sickness and every disease among the people. (Matthew 9:35)
Now after John was put in prison, Jesus came to Galilee, preaching the gospel of the kingdom of God, and saying, "The time is fulfilled, and the kingdom of God is at hand. Repent, and believe in the gospel." (Mark 1:14-15)
[Jesus] said to them, "I must preach the kingdom of God to the other cities also, because for this purpose I have been sent." (Luke 4:43)
Now it came to pass, afterward, that [Jesus] went through every city and village, preaching and bringing the glad tidings [gospel] of the kingdom of God. (Luke 8:1)
The law and the prophets were until John. Since that time the kingdom of God has been preached, and everyone is pressing into it. And it is easier for heaven and earth to pass away than for one tittle of the law to fail. (Luke 16:16-17)
From Jesus' own words, it is clear that the "good news" which Jesus Christ brought was about the Kingdom of God. The "gospel of Jesus Christ" is simply the message of good news that Jesus preached—not a message about Jesus. It is not primarily a message about the events in His life and of His becoming the Savior of the world—although it most certainly does include all that. But Jesus Christ's predominate theme in His "good news" announcement was the Kingdom of God.
The "Kingdom of God," though, has a dual application. For those who are not converted, the Kingdom of God represents the government of God which will be established on earth when Jesus Christ returns. With the perfect government—divine and infallible—ruling the nations, and with God's royal law being enforced as the standard of human conduct, man will experience peace, prosperity, and fulfillment as he never has in all his history.
But for the saints—those who belong to Jesus Christ—the Kingdom of God also represents their reward and their inheritance, including reigning on earth with Jesus Christ as kings and priests (Revelation 5:10). But flesh-and-blood mortals cannot inherit the Kingdom of God (I Corinthians 15:50). It is only when the saints are resurrected (or changed) at Jesus Christ's return that they will be made incorruptible and immortal, and thus able to inherit the Kingdom (I Corinthians 15:51-54; I Thessalonians 4:15-17).
This is what the context of I Corinthians 15:1-3 is about. The very next verse explains this part of the gospel message: ". . . and that He was buried, and that He rose again the third day according to the Scriptures" (I Corinthians 15:4). Taking the whole passage together, the gospel, then is not just that Jesus Christ died for our sins (verse 3), but that He died and was resurrected! Why is His resurrection so important? Because it is through the resurrection from the dead that one is born again—born into the Family (Kingdom) of God. It is only after being born of both water (baptism) and Spirit (being raised with incorruptible spirit bodies) that one enters the Kingdom of God (John 3:5-6).
Romans 8:29 gives Jesus Christ the title of "the firstborn of many brethren"—brethren who are conformed to His image (same verse). His brethren—Christians—will be born again, just as He was. How was He born? Through His resurrection from the dead. This is why He is called not just "the firstborn of many brethren," but also "the firstborn from the dead" (Colossians 1:18; Revelation 1:5). Just as Jesus was the first to be "born from the dead," so will His brethren likewise be born from the dead when they are resurrected at His return.
Jesus Christ's sacrifice plays a pivotal role, for without it there could be no fellowship with God and thus no relationship. But the message of hope which He brought was not merely about the forgiveness of sins. It was about what happens after that. His good news was about the Kingdom of God, which can only be born into through the resurrection from the dead. This is why Christ's resurrection is mentioned by Paul in conjunction with the gospel in I Corinthians 15:3-4. In fact, the entire rest of the chapter is about the resurrection from the dead and the hope that it offers to Christians.
Is Jesus Christ's death a fundamental part of the gospel message? Absolutely. But it is not the entire message.