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's important to realize that there's two sides to the "Jesus" question:

  1. Some guy named "Jesus" who existed.
  2. 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[1]. Suppose we didn't know about Saint Nicholas, but only "Santa Claus", if we later find out that Saint Nicholas was a historical figure, that doesn't mean that Santa Claus was delivering presents to billions of kids in 24 hours, breaking laws of physics with a gravity-defying sled pulled by flying deer.

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 [2]), and convinced some people around him, but for hundreds years, had extra supernatural claims added in.

It wouldn't take much historical evidence to demonstrate that a historical figure "Jesus", existed. Many Christians seem to think it's a package deal, though - that if we demonstrate that Jesus was real, that all 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's 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 eye-witness 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. That's because minds are very easy to fool, due to various cognitive glitches that are exploited.

It doesn't 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. Yet, unlike the Biblical "500 people", we can talk with, and interview, these people directly about what they saw. We don't have to rely on sparse broad claims about an unidentified group people who are long since dead, and who have no personal accounts recorded.

We'd have to drop the textual evidence, and engage in empirical experimentation - science - to evaluate whether the events were tricks, embellishment, or breaking the known laws of physics. Through properly-blinded, falsifiable, controlled studies and experiments, with peer review, we could investigate basis of the claims.

Until then, all we're left with is a basic probability. 500 people claim Jesus performed miracles, and resurrected. Which is more likely, and which possibility is the simplest, with the least assumptions (Occam's razor)?:

  1. Some "Jesus" guy could break the laws of physics and was the son of a universe-creating entity.
  2. They were mistaken.