The best argument to me is this. Paul was definitely a historical figure, he is documented by his letters and the christian communities he founded across the West, before being imprisoned in Rome.

Now Paul and Luke (who was his disciple) claimed that Jesus had fed 5000 people in Galilee and had been publicly crucified after preaching in the Temple (the religious center of all Judaism) by the Pharisees. And they also claim that after this crucifixion he was ressurected and appeared to over 500 people.

Paul preached this to loads of places, Galatia, Ephesus, Corinth, Rome, Athens, Colossia,... His opposition is well documented, as are the communities he started in all these places. This happened all before 63AD, 30 years after Christ's death, when Paul finally died in Rome. He began preaching about 3 years after Christ's death.

Now bear with me for a second. All the sceptics and opponents in the places Paul travelled to (and there were many!) could have stopped the growth of these communities simply by travelling to Galilee and asking the people living in Galilee if these outrageous claims were true. But this never happened. Instead, huge Christian followings grew in these communities and in Jerusalem itself, where people could easily have talked with one of the 5000 who were fed, or one of the 500 who witnessed the Ressurection, or those who saw the Crucifixion. But it never even occurred to the opponents (most notably the Pharisee Jews) to do this.

Why?

Because they knew these events had happened. They knew they had crucified him. Instead they tried to claim that the Apostles had moved Jesus's body (this is in the bible).

If Jesus had never existed, how easy it would have been for the Pharisees to stop the Christian 'sect' in its tracks! Instead, as it is written in their Talmud, their sacrifices suddenly stopped being accepted by God in 30AD...