Paul reminds the Corinthians the message he terms of first importance: Christ died, was buried, was raised, and appeared to witnesses including Paul himself.
Paul has received this gospel and is passing it on to the Corinthians. His knowledge is based on experience and scripture.
Now, back to us. We have had the opportunity of good, faithful teaching by hard-working teachers who were filled with grace and able to demonstrate that grace to us. And we have been faced with not-so-faithful, not-so-grace-filled teachings and examples. Let us hold firm to the good news proclaimed through Paul. Let us come to believe--and act as if we did. Let God's grace to us not be in vain.