The people who keep going on about eyewitnesses have evidently never studied the principles of historical method. There are plenty of historical figures of whom we have no eyewitness accounts, such as Pythagoras, Confucius, and Boudicca, but no-one questions their existence.
Josephus, for example, wrote about 60 years after the crucifixion. To dismiss his account would be like refusing to read a book about Hitler because it was written in the 1990s.
The New Testament is also an historical document, and its parts can be examined for evidence of authenticity, both internal and external. The evidence for Mark's gospel (excluding the addition at the end) and those Pauline letters that have a consistent style and vocabulary is quite good.
The upshot is that we can say there was a preacher called Jesus of Nazareth who was executed by the Romans. After his death his body vanished from his tomb and his followers, who had accepted him as the promised Messiah, claimed he had risen from the dead.
Note that the first Christians did not say that had seen him after his resurrection: Paul and the authentic text of Mark do not claim that and the other gospels cannot be shown to be by eyewitnesses.
Nor did they say he was the son of God. If they had, Josephus would have mentioned it, to say nothing of Mark and Paul. Paul even described Jesus as "physically" descended from David, which implies that he was the son of Joseph.
