What does 1 Corinthians 15:5 mean?

Paul is reminding the Christians in Corinth of what they believed when they became Christians. He is reminding them of the content of the gospel in order to show that it involves faith both in Christ's death on the cross and His resurrection from the dead. Apparently, the truthfulness of the resurrection, or at least the resurrection of Christians, was being challenged or misunderstood by some in Corinth. Paul wants them to understand not only that the resurrection really took place, but that faith in that truth is essential in order to be saved.

Now Paul begins to do two things. First, he wants to establish that he's far from the only apostle to teach that Jesus walked out of his tomb physically alive in a physical body. This same truth is taught by all the apostles, because they all saw Christ in person after He died.

Second, Paul wants to remind them of the historical reality of the resurrection. These first-century Christians did not need to clear too high of a hurdle to believe in Jesus' death and resurrection. Eyewitnesses still existed. They saw Jesus die, and they saw Him alive again, because Jesus revealed Himself to them.

First, Jesus showed Himself to Cephas—the original Aramaic name for Peter—and the rest of His inner circle of disciples. He didn't stop there, though.