In 1 Corinthians, the Apostle Paul summarizes the gospel like this:
… 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.1 Corinthians 15:1b-8A
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: And that 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, but some are fallen asleep. After that, he was seen of James; then of all the apostles. And last of all he was seen of me also…
Notice that the gospel is based on the simple fact that Christ died for our sins according to the scriptures. Not only this, but he was buried and that he rose again the third day.
Why is this considered to be the foundation of the gospel?
Simply because without Jesus Christ dying on the cross so many years ago, there would be no message of hope for mankind. In dying, he accepted the penalty of sin which should have been put on sinners, so that today, anybody can receive forgiveness of sins.
Will you believe on Him today?