This is the earliest written account of the resurrection of Jesus ON EARTH!
1 Corinthians 15:3-7 consists of a few unique lines of scripture that is the earliest documented account of the historical death, burial, resurrection and appearances of Jesus that we have on earth! And as such, these four verses are extremely important to studying the resurrection. Why? Because it reveals what the very first Christians believed and taught—it represents a creed, a formal statement of faith. Paul writes:
3 For I delivered to you as of first importance what I also received: that Christ died for our sins in accordance with the Scriptures, 4 that he was buried, that he was raised on the third day in accordance with the Scriptures, 5 and that he appeared to Cephas, then to the twelve. 6 Then he appeared to more than five hundred brothers at one time, most of whom are still alive, though some have fallen asleep. 7 Then he appeared to James, then to all the apostles.
The apostle Paul, a former zealous Pharisee, who received this oral creed, is saying he's faithfully passing it down just as he received it, likely from the apostles Peter and James, who he met in Jerusalem about 3-6 years after the crucifixion (which was approx. A.D. 30-33) and his famous conversion on the Road to Damascus. When Paul wrote this first letter to the Corinthians in about A.D. 54 or 55, this creed had already been circulating for years-- but, because the gospel narratives hadn’t even been written yet, this passage represents the very earliest documented account of the resurrection on earth!
But why does it matter that it was "early"? Because historians consider something that's written soon after the event more reliable than something written later. Logically, we're more likely to trust someone's account of a significant event in history, let's take 9/11 for example, the week it happened-- over someone writing now, decades later, about their recollection of what happened on September 11, 2001. The fact that this creed is early makes it a reliable account of history.
In fact, even the most skeptical of scholars agree on the early dating—with atheist Gerd Ludemann saying it dates within “the first two years after the crucifixion of Jesus,” and liberal Funk & The Jesus Seminar (sounds like the name of a groovy band!) recognizing it as being within “two or three years at most.”
So basically, with that kind of broad consensus, “we have what amounts to a certifiably official teaching of the disciples on the resurrection of Jesus.” Even if these five verses were the only lines of writing we had available from 2,000 years ago, given the early dating, we’d know the resurrection was not a late, legendary development, because we’d know exactly what the disciples testified to straight out of the gate: “This Jesus God raised up again, to which we are all witnesses.”
This is the "kerygma" or official proclamation, the heart of, and touchstone for the entire gospel message—which Paul says he receives and delivers to us “as of first importance.” This creed represents Christianity’s “foundation-story with which a community is not at liberty to tamper,” and so is “unalterable Christian bedrock.”
And the fact that it points to actual witnesses to this (and a cumulative number of them, to boot) means they were talking about a real event in time-space history, not just some kind of metaphorical or mythical resurrection complete with a loosey-goosey spiritual experience. Paul even says most of them are still alive, so feel free to go confirm this with them!
As a note, some skeptics point out that this creed doesn’t mention the empty tomb so maybe the apostles fabricated that fact and then inserted it later into the gospels. But the absence of the empty tomb in the creed is not significant because it’s simply a given. It’s implied. One scholar points out that the creed doesn’t need the phrase "and the tomb was empty" any more than the statement “I walked down the street” needs to be made qualified with the addition of “on my feet.” As a side observation, you’ll notice the cross isn’t mentioned in it either(!), but that’s because Jesus’ death by crucifixion was also a given.
It’s been pointed out that “mention of the empty tomb was reserved for the resurrection narratives, not creedal summaries.” Paul was passing on the highlight reel—the 4 bullet-points of the resurrection, crystallized in a confession– that Jesus died, was buried, rose again, and appeared.
Even now, some 2,000 years later, believers pass this important information on to others. He is risen; he is risen indeed!
 Galatians 1:18-20
 Acts 9
 Michael R. Licona, The Resurrection Jesus: A New Historiographical Approach (Downers Grove, IL: InterVarsity Press, USA, 2010), 234.
 Ibid., 232.
 Acts 2:32
 N.T. Wright, Christian Origins and the Question of God, Vol. 3, The Resurrection of the Son of God (Minneapolis, MN: Fortress Press, 2003), 319.
 Ibid., 317-8.
 Ibid., 321.
 Sean McDowell, “Was Paul Unaware of the Empty Tomb?” SeanMcDowell.org, April 13, 2019, https://seanmcdowell.org/blog/was-paul-unaware-of-the-empty-tomb