Is Christianity based on older myths?

[Mister Agenda]

You mean someone lied, lied, lied and lied. No one seems to want to tell me why the assumption is all these writers lied.

Because we have no obligation to defend the words you put in our mouths.

Or is it more proper to assume a writer of an ancient document is not being intentionally deceitful until it can be proven or at least a plausible explanation why the contents are false.

It's more proper not to assume either way. Revising history has been a popular human activity for as long as history has been recorded. Without independent corroboration, we can't evaluate whether a given author is more likely to be honest or dishonest. And even the dishonest ones are likely to include some actual facts and even the honest ones are going to get some things wrong. And that's without going into the issue of unconscious bias.

There are at least 8 authors of these 27 docs.

Yep, at least.