I recently was told by an atheist this about 1 Corinthians 15:3-7:

What Paul reports in 1 Corinthians is not an eyewitness account – nor does he claim it to be. He says he ‘received’ it. A believer posted on my site that she often receives things from God – she hears his voice. I asked if it was an audible voice, and she said, yeah – sort of.

If eyewitness doesn’t mean witnessed with the eye, and audible doesn’t mean heard with the ear – then I don’t really know how to have a reasonable conversation with believers. Folks like this are ultimately going to believe whatever they want, no matter what.

I sputtered for a while at the suggestion that Paul’s early creed in 1 Cor 15:3-7 was an auditory hallucination and not an early tradition received from the eyewitnesses, but then I calmed down and wrote something sensible, so here it is, without quotes. (Keep in mind that I was very angry when I wrote this because… GAAAHHHH!!!)

Note: I should be clear and say that skeptics accept that the people who report having appearances had some sort of experience where they thought they saw Jesus. Gerd Ludemann calls them “visions”, for example.

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If you study these things, you will find that the tradition in 1 Corinthians 15 is accepted by atheistic scholars like James Crossley as going back to 1-3 years after the death of Jesus. This is an early creed that was received from the eyewitnesses Peter and John when Paul visited them several times in Jerusalem, as documented in Galatians 1 and 2, where Paul meets the eyewitnesses. And of course, Paul has his own eyewitness experience, documented in 1 Cor 15:8. This account is admitted by atheistic scholars on the far left fringe. Even the Jesus seminar colors it red in their “The Five Gospels” book. It is the only way to explain Paul’s conversion. He was an eyewitness. He met with the eyewitnesses.

Here is an introduction:
http://www.reasonablefaith.org/the-witness-of-the-pre-pauline-tradition-to-the-empty-tomb

The evidence that Paul is not writing in his own hand in I Cor. 15.3-5 is so powerful that all New Testament scholars recognize that Paul is here passing on a prior tradition. In addition to the fact that Paul explicitly says as much, the passage is replete with non-Pauline characteristics, including, in order of appearance: (i) the phrase “for our sins” using the genitive case and plural noun is unusual for Paul; (ii) the phrase “according to the Scriptures” is unparalleled in Paul, who introduces Scriptural citations by “as it is written”; (iii) the perfect passive verb “has been raised” appears only in this chapter and in a pre-Pauline confessional formula in II Tim. 2.8; (iv) the phrase “on the third day” with its ordinal number following the noun in Greek is non-Pauline; (v) the word “appeared” is found only here and in the confessional formula in I Tim. 3.16; and (vi) “the Twelve” is not Paul’s nomenclature, for he always speaks of the twelve disciples as “the apostles.”

Now the visit during which Paul may have received this tradition is the visit you mention three years after his conversion on the road to Damascus (Gal. 1.18). This puts the tradition back to within the first five years after Jesus’ death in AD 30. So there’s not even an apparent inconsistency with Paul’s appropriating the language of the formula to encapsulate the Gospel he was already preaching during those first three years in Damascus.

I have heard atheists accept the 1 Cor 15 creed as having that early date in debates with Craig.

Here are a few of them:
http://www.garyhabermas.com/articles/dialog_rexperience/dialog_rexperiences.htm

Quote:

(1) Contemporary critical scholars agree that the apostle Paul is the primary witness to the early resurrection experiences. A former opponent (1 Cor. 15:9; Gal. 1:13-14; Phil. 3:4-7), Paul states that the risen Jesus appeared personally to him (1 Cor. 9:1; 15:8; Gal. 1:16). The scholarly consensus here is attested by atheist Michael Martin, who avers: “However, we have only one contemporary eyewitness account of a postresurrection appearance of Jesus, namely Paul’s.”[3]

(2) In addition to Paul’s own experience, few conclusions are more widely recognized than that, in 1 Corinthians 15:3ff., Paul records an ancient oral tradition(s). This pre-Pauline report summarizes the early Gospel content, that Christ died for human sin, was buried, rose from the dead, and then appeared to many witnesses, both individuals and groups.

Paul is clear that this material was not his own but that he had passed on to others what he had received earlier, as the center of his message (15:3). There are many textual indications that the material pre-dates Paul. Most directly, the apostle employs paredoka and parelabon, the equivalent Greek terms for delivering and receiving rabbinic tradition (cf. 1 Cor. 11:23). Indirect indications of a traditional text(s) include the sentence structure and verbal parallelism, diction, and the triple sequence of kai hoti Further, several non-Pauline words, the proper names of Cephas (cf. Lk. 24:34) and James, and the possibility of an Aramaic original are all significant. Fuller attests to the unanimity of scholarship here: “It is almost universally agreed today that Paul is here citing tradition.”[4] Critical scholars agree that Paul received the material well before this book was written.[5]

The most popular view is that Paul received this material during his trip to Jerusalem just three years after his conversion, to visit Peter and James, the brother of Jesus (Gal. 1:18-19), both of whose names appear in the appearance list (1 Cor. 15:5; 7). An important hint here is Paul’s use of the verb historesai (1:18), a term that indicates the investigation of a topic.[6] The immediate context both before and after reveals this subject matter: Paul was inquiring concerning the nature of the Gospel proclamation (Gal. 1:11-2:10), of which Jesus’ resurrection was the center (1 Cor. 15:3-4, 14, 17; Gal. 1:11, 16).

Critical scholars generally agree that this pre-Pauline creed(s) may be the earliest in the New Testament. Ulrich Wilckens asserts that it “indubitably goes back to the oldest phase of all in the history of primitive Christianity.”[7] Joachim Jeremias agrees that it is, “the earliest tradition of all.”[8] Perhaps a bit too optimistically, Walter Kasper even thinks that it was possibly even “in use by the end of 30 AD . . . .”[9]

Indicating the wide approval on this subject, even more skeptical scholars frequently agree. Gerd Ludemann maintains that “the elements in the tradition are to be dated to the first two years after the crucifixion of Jesus. . . . not later than three years. . . . the formation of the appearance traditions mentioned in I Cor.15.3-8 falls into the time between 30 and 33 CE. . . .”[10] Similarly, Michael Goulder thinks that it “goes back at least to what Paul was taught when he was converted, a couple of years after the crucifixion.”[11] Thomas Sheehan agrees that this tradition “probably goes back to at least 32-34 C.E., that is, to within two to four years of the crucifixion.”[12] Others clearly consent.[13]

Overall, my recent overview of critical sources mentioned above indicates that those who provide a date generally opt for Paul’s reception of this report relatively soon after Jesus’ death, by the early to mid-30s A.D.[14] This provides an additional source that appears just a half step removed from eyewitness testimony.

(3) Paul was so careful to assure the content of his Gospel message, that he made a second trip to Jerusalem (Gal. 2:1-10) specifically to be absolutely sure that he had not been mistaken (2:2). The first time he met with Peter and James (Gal. 1:18-20). On this occasion, the same two men were there, plus the apostle John (2:9). Paul was clearly doing his research by seeking out the chief apostles. As Martin Hengel notes, “Evidently the tradition of I Cor. 15.3 had been subjected to many tests” by Paul.[15]

These four apostles were the chief authorities in the early church, and each is represented in the list of those who had seen the resurrected Jesus (1 Cor. 15:5-7). So their confirmation of Paul’s Gospel preaching (Gal. 2:9), especially given the apostolic concern to insure doctrinal truth in the early church, is certainly significant. On Paul’s word, we are again just a short distance from a firsthand report.

(4) Not only do we have Paul’s account that the other major apostles confirmed his Gospel message, but he provides the reverse testimony, too. After listing Jesus’ resurrection appearances, Paul tells us he also knew what the other apostles were preaching regarding Jesus’ appearances, and it was the same as his own teaching on this subject (1 Cor. 15:11). As one, they proclaimed that Jesus was raised from the dead (15:12, 15). So Paul narrates both the more indirect confirmation of his Gospel message by the apostolic leaders, plus his firsthand, direct approval of their resurrection message.

Now, some of the people he lists there are people who I think are so far on the secular left that they are on the far left fringe to the point of being irrational – they are so biased. And yet they don’t deny 1 Cor 15, or that it is based on eyewitness accounts.

That why you see people like Bart Ehrman, Gerd Ludemann, James Crossley, Michael Goulder, etc. giving Craig the appearances as a historical fact in debates. Atheist or not, people who deny the resurrection accept the appearances listed in this creed, and they think it is based on eyewitness testimony, including Paul’s. That’s the way it is. That’s what you read in the papers. That’s what you see in debates. This is a given.

That’s what I mean when I say they are non-negotiable. I mean that scholars on the far left secular skeptical fringe consider them to be non-negotiable.