Upon what did the Galatians, Romans, Corinthians, and Thessalonians believe their faith was founded. Did they think (1) that the gospel message was based on the teachings of a first century itinerant Rabbi that had been passed along by the original disciples who had been taught by that Rabbi, or did they think (2) that the gospel message was something that had been directly revealed to Paul by a divine being? If we only had Paul's writings to go on, we would have to conclude that Paul's revelation were the source of his followers' beliefs.
I have raised this point numerous times with both liberal and conservative Christian who have offered various reason why Paul's letters never disclose the fact that the source of the gospel he preached was the teachings of the historical Jesus rather than the revelations of the divine Christ. The most common explanation boils down to "It just never came up." According to this line of thinking, the epistles Paul wrote were directed towards issues that never required him to say anything to indicate when or where Jesus lived or what he said or did during his life. Another explanation lies in the competitive tension between Paul and Peter. Paul was trying to establish his authority in various theological disputes with Peter and acknowledging that Peter and others had been taught directly by Jesus would have diminished Paul's standing. I don't find either explanation terribly convincing, but even if I did, they would still leave a basic problem unaddressed.
These explanations leave open the possibility that Paul and his followers did believe that Jesus was a recently deceased miracle working teacher, but they don't give us any reason to think they did. Paul letters are the earliest Christian writings and our best source for understanding the early church. They tell us the kind of questions that were in dispute in the early church, and the kind of arguments that were considered dispositive of those questions. We shouldn't expect these letters to include absolutely everything that Paul and his followers believed about Jesus, but for those beliefs not reflected in the letters, we need some reason to think they were a part of the early faith.
One reason to think that Paul and his followers believed something would be to show it to be a generally held belief during their time. After all, Paul acknowledges that there were others preaching the gospel at the same time he was. If a belief can be shown to held by Paul's contemporaries within the Christian community, it would seem likely that it was part of Paul's faith, too.
When we look at the other early epistles, we don't find much evidence of others thinking that Jesus was a first century teacher whose message was spread by his original disciples. Like the Pauline epistles, the pseudo-Pauline and the Johannine epistles, along with Hebrews and James are focused on the supernatural risen Christ to the almost complete exclusion of a human Jesus who actually walked the earth. Only when we get to later epistles like 1 Timothy and 2 Peter do we get any indication that people known to the authors had personal knowledge of the things Jesus did prior to his crucifixion.
The gospels of course do describe the human Jesus as someone who initiate the proclamation of the gospel, worked signs and wonders, and taught his original disciples the meaning of his life, and coming death and resurrection, but they are nonetheless problematic. The date of their composition cannot be established with any confidence. Their authors are unknown. The place of their composition and their intended audiences are unknown. The specific questions that they were intended to resolve are much less certain than those addressed by the epistles. The extent to which they were meant to be understood historically rather than theologically is not clear. Perhaps the biggest hurdle is the lack of unambiguous external references to the gospels until well into the 2nd century. Even if we accept dates for their composition that are nearer to Paul's time, we cannot establish that they were generally accepted or in general circulation until much later.
Even if we conclude that Paul believed that Jesus had been a flesh and blood human who walked the earth prior to his crucifixion, there does not seem to be any way to establish that Paul or his followers thought of him as the 1st century preacher described in the gospels. Paul could have thought of Jesus as someone who had lived at a indeterminate time and place like Job in the Old Testament. Paul does tell us that he knew others to whom the risen Christ had earlier appeared, so it might be reasonable to think that he believed others had received earlier revelations, but there doesn't seem to be any evidence that Paul thought that anyone he knew had received any teachings from the human Jesus during an earthly ministry.