What is the Gospel? According to 1 Corinthians 15:1-4, the Gospel is embodied in these words: "Moreover, brethren, I declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand; By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain. For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures; And that he was buried, and that he rose again the third day according to the scriptures."
1 Corinthians 2:2 says, "For I determined not to know any thing among you, save Jesus Christ, and him crucified."
Warren Wiersbe writes in The Bible Exposition Commentary, "Proof #2 of Jesus' resurrection is the Old Testament Scriptures. 'First of all' means 'of first importance.' The Gospel is the most important message that the church ever proclaims. While it is good to be involved in social action and the betterment of mankind, there is no reason why these ministries should preempt the Gospel "Christ died... He was buried... He rose again... He was seen' are the basic historical facts on which the Gospel stands. 'Christ died for our sins' is the theological explanation of the historical facts. Many people were crucified by the Romans, but only one 'victim' ever died for the sins of the world. When Paul wrote 'according to the Scriptures,' he was referring to the Old Testament Scriptures. Much of the sacrificial system in the Old Testament pointed to the sacrifice of Christ as our substitute and Saviour. The annual Day of Atonement and prophecies like Isaiah 53 would also come to mind. But where does the Old Testament declare His Resurrection on the third day? Jesus pointed to the experience of Jonah. Paul also compared Christ's Resurrection to the 'firstfruits,' and the firstfruits were presented to God on the day following the Sabbath after Passover. Since the Sabbath must always be the seventh day, the day after Sabbath must be the first day of the week, or Sunday, the day of our Lord's Resurrection. This covers three days on the Jewish calendar. Apart from the Feast of Firstfruits, there were other prophecies of Messiah's Resurrection in the Old Testament."