In 1 Corinthians 15, Paul reminds us what the original Gospel is, the Gospel of Jesus Christ. He said that if the Gospel was forgotten, they had believed in vain.
Already, some had forgotten what the Gospel was. They believed that Jesus was the Christ and that He died for their sins, but had forgotten the Gospel that was given to the saints, the Good News that Paul dedicated his life to spreading.
Paul reminded them of the Gospel that he had preached to them. If they held fast to the Gospel of Jesus Christ, received it in their hearts and stood for it, they would be saved.
It was not difficult to remember. Paul said that the Gospel of Jesus Christ was the first and most important knowledge that he received. And it was simple: Christ died for our sins in accordance with the scriptures. He died and was buried. He rose from the dead on the third day, just as the Scriptures proclaimed. This is the Gospel that Paul said we are to believe, and this Gospel gives eternal life.
Christ did not remain in the grave. He appeared to Simon Peter and the twelve apostles. He appeared to more than five hundred brothers at one time, most of which were still alive while Paul was spreading the Good News. Then Christ appeared to James and all of the apostles.
Paul said that last of all, Christ appeared to Paul himself. He was untimely born, unworthy to be called an apostle because he persecuted the church of God.
But by the grace of God, Paul said, "I am what I am." By God's grace, Paul said that he was able to work harder than any of them in spreading the Gospel. It was not because of anything Paul could do in of himself, but by the Grace of God that was with Paul.
Paul said it did not matter who they received the Gospel of Jesus Christ from. Whether it was the apostles or himself, they preached the Gospel so that the people could believe the Gospel.
As Christians, we should ask ourselves: Do we remain founded in the Gospel of Jesus Christ, the Good News that Christ came, lived a perfect life and died for our sins, was buried and conquered death and hell, and then rose again so that we can rise with Him? Do we even know what the Gospel is? Have we forgotten that Christ died for sinners as well as saints? Are we a believer of the Gospel of Jesus Christ, spreading it everywhere we go, or are we spreading something else?