[tsimionescu]

That would make it unknowable whether he existed, it would not mean we have reason to believe he didn't.

This is not like physics where it's natural to assume something didn't exist if the proof for its existence is not strong enough.

[ncmncm]

Only because a lot of people want to believe.

We are all confident Adam, Noah, and Moses were made up, with none of the proof positive that you are demanding for this one case.

[tsimionescu]

Adam, Noah and Moses were only attested and believed to exist by groups living in the Kingdom of Judah.

For Moses, people have looked a lot for any kind of evidence that there was some significant Jewish presence in Egypt, or Egyptian migration to the area of Israel, and nothing of the kind has been found, either in Egyptian documents or in archaeological evidence - which is actually evidence of absence when expecting a significant population to have migrated that way. Further accounts from the books of Exodus and Deuteronomy, such as the battle of Jericho, have also been somewhat conclusively debunked (the city of Jericho hadn't existed, at least not worth walls, for a few hundred years before the time of the conquest is supposed to have taken place).

Similarly, we have looked long and hard for evidence of a massive flood that could have lent some credence to the story of Noah, and nothing of any significant magnitude was found for that time - and here, we know for sure that a flood would have left significant geological evidence, so we know the flood can't have existed.

Adam has so little information associated with him that it's hard to even define what it would have meant for him to exist. We do know for sure, based on DNA evidence, that there is no single father + mother pair from which all humans living today have sprung, definitely not anyone living anywhere near close to the Jewish account of Adam.

In contrast, the idea of a founder of the Christian sect, one who was killed under Pontius Pilate around the year 33, has no major evidence against it, and is a somewhat plausible account of how the Christian sect could have come to be. There are no sources asserting a different origin, and there are no sources that contradict the possibility that Pontius Pilate and the Jewish authorities would have punished someone behaving like Jesus did. So, the neutral position is to say that he may or may not have existed, we don't know.

If you further believe the biblical or non-biblical sources attesting to his existence, even if you think they are weak, you can even say that it's more likely that he existed than that he didn't.

[ncmncm]

No one who lived then and wrote anything about Jesus, including the (unknown) authors of the Gospels, ever claimed to have met Him.

We can be confident Paul existed, or anyway somebody we know of as Paul, who wrote his Letters. Likewise Homer, the Iliad. Tacitus, Pliny, Horace, Plato, Euripides. But there is nothing traceable to any Jesus. You certainly can choose to believe He existed, but objectively, the evidence is too thin to support it.

Funny thing about Noah's flood. The water is all still there. We call it the sea. Sea level rose 120 meters in the past 20,000 years, up until 8000 years ago. Many millions of square miles of what was rich river bottom land is now sea floor. People whose family had lived there for tens of thousands of years had to keep moving inland (where other people already lived!) as the sea swallowed their ancestral homes. For 12000 years. It must have made an impression.