Paul reminds the Corinthians of the good news that saves us if we continue to believe it. What was most important? That Christ died for our sins, was buried and was raised from the dead – all as the Scriptures had said. The risen Jesus was seen by Peter, the twelve, and more than 500 at one time – many of whom were still alive when Paul wrote this letter. He was seen by James, the apostles, and finally by Paul.
The crux of our trust in Jesus is the resurrection. Without it, our faith is useless – and worse, we would be making liars out of God and still be guilty of all of our sins: we would still be lost!