"I handed on to you as of first importance what I also received..." If the rest of 1 Cor 15:3-5 is the tightly compressed summary of the Christian story—that he died for sins, was buried, was raised, and was seen—then this introductory line tells us quite a lot too. 1) This is of first importance; this is the stuff you need to know above all else. 2) Paul handed it on to them, first in person, now in this letter. What he is reminding them now (verse 1) is the gospel he had preached, and the verb for "handing on" is where we get "tradition", just like in "Therefore, brothers, stand firm and hold fast to the traditions you were taught, whether by word of mouth or by letter from us." The gospel, the good news, the tale of the paschal mystery, is the Apostolic Tradition that we hold fast to and build our lives around.