The Romans kept a lot of records, and from this is historical proof that a man named Jesus was born in Bethlehem and crucified outside Jerusalem.
Whether or not you believe he was the Son of God is up to you, but you can't say he definitely did or didn't perform miracles. So the reason some athiests accept the records of this man is because they believe he did exist but was just a man.
It is historically accepted (generally) that Jesus both existed AND did great feats that were recorded. I am not saying it is accepted by most that he did them by the power of God but even his contemporary opponents accepted these two things. Josephus writes on this. Things like healings are attributed to him and are NOT academically attributed as lies or hysteria, the usual explanation is his charisma fixed there phsycological impairments. His teaching is also attributed to have come from him. Academically there is no sound reason not to believe he existed and taught (most of) what we now see him teach in the bible and that he did many miracles. Please don't (anyone) post things like he was mentally deficient/crazy as smart people of this era are in agreement that he was not, also do not post that he was just a nice guy people liked enough to follow; he was a respected leader among many true but his teaching was controverial and caused genuine anger. He could easily have been more popular by not giving such "difficult teaching" that turned people away and enraged them to the point of killing him. His teaching was effective because it was different, true and challenging. He also taught in different ways to the rabbis of the time and lived the life he taught on. This is generally accepted among those who study Jesus.(at least according to the doctor of Theology i've listen to lecture on such things, and no i have no reason to suspect her a Christian and the course is at least not geared towards Christians) Oh it is also accepted he was killed by crucifiction, Romans do there jobs well, the argument he was still alive afterwards is laughable.
He was an actual living human being. There is plenty of proof about that from historical writings and events.
Just the celebrity if his time.
It was never even proposed that he was the son of God until they took a vote on it at the council of Nicea. Most Christians don't realise that their beliefs are based entirely on the vote of a council. What if they had voted differently?
I believe he existed because there were hundreds of similar 'messiah'-types around at the same time. People need to understand the sociological climate when Christianity was born - there was a veritable supermarket of faiths, will hundreds of varying sects believing their own amulets and passcodes would get them through to heaven. Christianity caught on mainly because it became compulsory to do so!
Plus, as someone said above, lots of people died for Jesus. And not long after Jesus' death.