Atheists vary in how much they already accept that Jesus was a historical figure. Some believe it, some reject the notion, and some are on the fence.
It is important to realize that there's two sides to the "Jesus" question:
- Some guy called "Jesus" existed.
- The supernatural claims (son of god; performed miracles) were true
While some atheists accept #1, none accept #2.
Consider the concept of "Santa Claus", who is loosely modeled after Saint Nicholas, a 4th-century Christian saint. Supposing we didn't know about Saint Nicholas, but only Santa Claus, if we later find out that Saint Nicholas was a historical figure, that does not mean that Santa Claus was delivering presents to billions of kids in 24 hours, breaking laws of physics.
It means that Santa Claus was "based on a true story", and embellished significantly. Likewise, "Jesus" may have been based on a true story of some guy who thought he was a messiah (self-proclaimed messiahs are a dime-a-dozen ), and convinced some people around him, but for a few hundred years, had extra supernatural claims added in.
The supernatural Jesus could be the biggest growing-fish story of all time.
It wouldn't take much historical evidence to demonstrate that a historical figure "Jesus", existed. Many Christians seem to think it is a package deal, though - that if we demonstrate that Jesus was real, that the supernatural claims are automatically true/demonstrated too.
Textual evidence (such as the Bible) may be sufficient to demonstrate a mundane claim, like a historical figure existing, but it is woefully insufficient to demonstrate extraordinary claims, such as the supernatural. How could "because a book said so" be adequate to demonstrate that someone could break the laws of physics? Not even personal testimony could do this.
In the modern world, we have a career called "professional magician" (many of who are atheists), who manage to trick/deceive most of an entire audience into thinking that supernatural events just occurred.
It does not matter if we had one person, or 300,000 people, who come out of that audience saying they saw a miracle, all we can derive from their testimony is that some kind of event occurred where they think supernatural events took place.
... and we could talk with and interview those people directly about what they saw. We don't have to rely on a unidentified group of "500 people", like the 2000 year-old Bible claims, who are long since dead.
We'd have to drop the textual evidence, and engage in empirical experimentation - science - to demonstrate that the claimed supernatural events were even real, and not just tricks or embellishments. Through properly blinded, falsifiable, controlled studies and experiments, with peer review, we could investigate the claims.
Until then, all we are left with is a basic probability. 500 people claim Jesus performed miracles, and resurrected. Which is more likely - which possibility is the simplest, and requires the least assumptions (Occam's razor)?:
- Some "Jesus" guy could break the laws of physics and was the son of a universe-creating entity.
- They were mistaken.