The salvation of some was in jeopardy. By this gospel you are saved, if you hold firmly to the word I preached to you. Otherwise, you have believed in vain. After so much struggle, after what Jesus had done for them, the last thing Paul wanted for the Corinthians would be that their faith had been in vain. Paul understood that time can blur the vision.
Priorities can become confused.
Apathy can set in. Paul wrote to fight against such corrosion, praying that the Holy Spirit would give him the words the church needed to hear. For Paul, the gospel did not leave Jesus on the cross. It also raised him from the grave. Paul supported his testimony of Jesus' resurrection by listing many of its eyewitnesses.