We have pretty good records of letters from Paul from the second half of the first century (as well as a bunch of frauds, but AFAIK there are several that stand up under analysis as being written by the same person at a very early date), who (in several of those letters) was adamant that his sect believed in an individual Christ in the flesh. So if the legend of the historical Jesus sprang from whole cloth, it did so pretty quickly- not to mention the synoptic gospels, which the most skeptical of scholars still date to around 100 AD.

More generally, I'd caution fellow atheists against getting drawn into the existence-of-Jesus debate in meatspace: unless you've done a lot of relevant study (and why on earth would you?) you won't be able to point to basic evidence that your interlocutor has heard of, only to the statements of experts that they won't trust. Better to say "Well, I'm not sure there's enough evidence even to conclude that there was a historical Jesus- but even granting that there was, and that there came to be a group of followers convinced of his divinity, that's still nowhere near the kind of evidence to make Christianity a viable hypothesis, compared to the hypothesis that it was just a rabidly successful example of what happens within cults." That's a much better place to draw up battle lines, IMO.