Throughout the New Testament, one will find early creeds, formulations, and hymns that predate the New Testament itself. These texts are often called “proto-New Testament texts.” Proto-New Testament texts date back to the earliest church from those who were eyewitnesses of Jesus himself. 1 Corinthians 15:3-8 is perhaps the most popular of the proto-New Testament texts as it bears heavily on the post-resurrection appearances of Jesus. Concerning 1 Corinthians 15:3-8, Roy Ciampa and Brian Rosner write,
Paul had used the language of “receiving” and “passing on” traditions in 11:23 with respect to the Lord’s Supper. The information about the gospel had been passed on as being of first importance. While the expression could mean “at first” (and Paul undoubtedly shared this with the Corinthians very early on in his ministry among them), the nearly unanimous preference of English translations (first in importance) is probably correct.
The fact that he had received this information about Christ does not contradict his point in Galatians 1:12 that he received his gospel “by revelation from Jesus Christ.” While the basic gospel message was received by revelation from the Lord, the formulation he used in preaching the gospel included elements that had been passed on to him by those who were Christians before him, perhaps including the fact that Christ died for our sins and that it was according to the Scriptures, that his resurrection took place on the third day and that that was also according to the Scriptures, and the information about the witnesses to Christ’s resurrection.