Here is one piece of evidence for the existence of an actual historical Jesus: Paul claims to have confronted Peter in person in Galatians, which is one of the Pauline epistles considered to be genuine. This is solid evidence for Peter being a real person. Peter is supposed to have been one of Jesus's disciples. So how can we plausibly claim that early Christianity did not have a Jesus? This is the same level of evidence for many Greek philosophers being real people.
https://en.wikipedia.org/wiki/Historicity_of_Jesus
https://en.wikipedia.org/wiki/Richard_Carrier#Reception_and_...