I think it is important to note that the records of Jesus' resurrection appearances are part of Paul's gospel.
• Jesus appeared:
o To Cephas (Peter)
o To the Twelve
o To more than 500
o To James
o To all of the apostles
o To Paul (as of one untimely born).
The gospel is tied up with the experience of a human being who literally died and who came back to life and appeared to human beings. This points to the objective reality of the Resurrection. He “was raised” and he “was seen.” Paul’s point seems emphatic. The resurrection of Jesus was not a form of “spiritual” existence. This was not a vision experience. Just as he was truly dead and buried, so he was truly raised from the dead bodily and seen by a large number of witnesses on a variety of occasions.
The appearance to James, how significant was that?
John 7.2-5 – But when the Jewish Feast of tabernacles was near, Jesus’ brothers said to him, “You ought to leave here and go to Judea, so that your disciples may see the miracles you do. No one who wants to become a public figure acts in secret. Since you are doing these things, show yourself to the world.” For even his own brothers did not believe in him.
Galatians 2.9 – “James, Peter and John, those reputed to be pillars, gave me and Barnabas the right hand of fellowship when they recognized the grace given to me.” James, the former non-believer and scoffer, is now mentioned as a pillar and listed with two of Jesus’ most prominent apostles, Peter and John.
I think it is important that Paul mentions this appearance to James. I believe that James was not a believer in Jesus until he encountered the resurrected Jesus. This is what changed James. The scoffer becomes the devoted.
Paul’s appearance? Acts 9.1-15, this was no mere vision experience. Paul puts it in the same league with Jesus’ appearances to Peter and the 12 (John 20, Luke 24.36-42).
Back up: believe in vain? This gospel message of Christ dying for our sins and, literally, being raised to continued life, is what we count on as true and effective and we must hold fast to it. We must remain in it or it will be useless to us. If we do not believe this, then our faith is useless (see 1 Corinthians 15.12-19).
The Gospel in 1 Corinthians 15: Christ, literally, died for our sins (in accordance with previous revelation) and was raised and physically appeared to his followers. This gospel message saves us. If it is not true (specifically the resurrection of Jesus), then our faith is useless and we are still in our sins.