What does 1 Corinthians 15:8 mean?

Paul is emphasizing to the Corinthians that to believe the gospel means believing in both the death and resurrection of Christ. This requires faith, yes, but Paul is making the point that the faith of the Corinthians is not a blind faith. After Jesus died, Paul has written, He was seen alive again by Cephas—the apostle Peter, whom the Corinthians knew. He was seen by the twelve disciples trained and sent out by Jesus, and more than 500 eyewitnesses at one time. Jesus also appeared to His half-brother James and even more eyewitnesses. Most of those were still alive at the time this letter was written, meaning that they continued to be available to describe it.

Finally, Paul now writes, Christ appeared to him. Paul describes himself as someone who was "untimely" or "abnormally" born. He is likely referring to being born again, coming into relationship with Christ, after all these others had come to know Him during Jesus' time on earth. Paul only came to faith in Christ after Jesus had already ascended back to heaven, when the Lord appeared to Him on the road to Damascus (Acts 9:3–6; 22:6–11). In that way, Paul is a prototype of the modern believer: one who learns of Christ through the resurrection, looking back, as opposed to the other apostles, who learned of Christ before the resurrection.

Paul includes himself among the eyewitnesses to Jesus' post-death life, meaning that it really was the Lord "in the flesh" who confronted Paul's unbelief in Christ's deity and the resurrection.