“At the time” was a few hundred years after his death. There isn’t even Roman records for him, let alone his crucification. Christianity also underwent several drastic dogma shifts between their early years and what eventually came out a thousand years later, a Christian today would barely recognize an early Christian’s beliefs at all. None of this is unique to Christianity of course, all religions undergo similar evolutions.
Scholars generally accept that the book of Mark was written between 66-70 AD. The idea that the gospels were written "hundreds of years later" is a common myth on the Internet that scholars do not take seriously. It is clear that Jesus's followers believed in his resurrection, they proclaimed it loudly and suffered and died for it, gaining many converts.
The parent comments point is that it could have been regarded as fiction at the time, but that was lost over time. If that were the case, we would not know that it was ever considered to be fiction.