"if Jesus did not exist, how come within a few decades of his death" is logically incoherent, of course - if he didn't exist then there's no "within decades of his death".
Ignoring that, "people make things up" is a perfectly believable answer. Fiction is a well-known example; many people write about Batman, but there's no such person. Religion is another; consider Scientology if you doubt that religions get invented plain and simple. The urge to invent isn't modern, either; ancient historical writers often put lengthy speeches into the mouths of their characters, which are pretty obviously made up.
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"if Jesus did not exist, how come within a few decades of his death" is logically incoherent, of course - if he didn't exist then there's no "within decades of his death".
Ignoring that, "people make things up" is a perfectly believable answer. Fiction is a well-known example; many people write about Batman, but there's no such person. Religion is another; consider Scientology if you doubt that religions get invented plain and simple. The urge to invent isn't modern, either; ancient historical writers often put lengthy speeches into the mouths of their characters, which are pretty obviously made up.