> Critical analysis has shown that the Gospels are written from the same linguistic and cultural perspective as their contemporaries.
That's not my point. My point is that you can't argue about the Gospels based on what was the case in oral tradition, because the Gospels aren't oral tradition. They're literature. Considering literature is in fact easier to preserve intact than oral tradition (that's precisely why we write things down), I'm not even sure why you'd appeal to oral tradition as though it were a source of reliability anyway.
> By what measure are you concluding that the Gospels are unreliable?
First, because the Gospels don't even use the recognizably imperfect standards of ancient historians anyway. Yes, people like Tacitus are generally reliable, but none of the Gospels use any of the methods employed by Tacitus. None of the Gospels tells us the name of their author; that we know Tacitus wrote the Annals because he put his name to the document is more than we have for any Gospel. At no point do the Gospels name their sources, use those sources comparatively, discuss the relative merits of those sources, or explain why they are reliable. They do not discuss methods, or the possibility that their information is incorrect, or the existence of any alternative accounts. They express no amazement at anything they report, no matter how fantastical it is. They never acknowledge when they have changed what their sources say, or explain why they did so. They do not read like eyewitness accounts or collections of such, nor do they identify themselves that way.
Second, because many scholars have demonstrated that the Gospels are primarily and pervasively mythical. Required reading on this topic:
John Dominic Crossan, The Power of Parable
Randel Helms, Gospel Fictions
Dennis MacDonald, The Homeric Epics and the Gospel of Mark
Thomas Thompson, The Messiah Myth
Thomas Brodie, The Birthing of the New Testament
In the words of Marcus Borg, in Jesus: Uncovering the Life, Teachings, and Relevance of a Religious Revolutionary, we have to admit "(1) that much of the language of the Gospels is metaphorical; (2) that what matters is the more-than-literal meaning and (3) that the more-than-literal meaning does not depend upon the historical factuality of the language."
We know that many of the narratives are rewrites of pre-Christian Jewish stories. Matthew borrows stories about Moses to turn into stories about Jesus, and borrows from Daniel to rewrite Mark's empty tomb narrative. Mark created the crucifixion narrative largely from Psalm 22.
Mark 15.24:
And they crucified him, and divided his clothes among them, casting lots to decide what each should take.
Psalm 22.18:
they divide my clothes among themselves, and for my clothing they cast lots.
Mark 15.29-31:
Those who passed by derided him, shaking their heads and saying, “Aha! You who would destroy the temple and build it in three days, 30 save yourself, and come down from the cross!” 31 In the same way the chief priests, along with the scribes, were also mocking him among themselves and saying, “He saved others; he cannot save himself.
Psalm 22.7-8:
All who see me mock at me; they make mouths at me, they shake their heads; 8 “Commit your cause to the Lord; let him deliver— let him rescue the one in whom he delights!”
And of course, Mark 15.34 and Psalm 22.1:
“My God, my God, why have you forsaken me?”
We can find examples of Jewish rabbinical legends rewritten into the Gospels. Inventing stories about famous people was the norm, and students educated in Greek schools (which the authors of the Gospels obviously would have been, writing in Greek) were explicitly taught how to do so, crafting symbolic narratives out of general proverbs and making major or minor changes to traditional stories to make whatever points they saw fit. They were also taught to emulate old stories, changing their messages along the way, with new characters and new outcomes; Homer wrote about a boxing match between Epeius and Euryalus, which Virgil turned into a boxing match between Dares and Entellus which changed the outcome and promoted Roman values of wisdom and experience as superior to Greek glorification of youth and vigor.
> Several of the Gospel writers specifically say that objectivity and accuracy are their agenda, so you need to support your claim.
No, they don't. But regardless, even if they did, the failure to actually write as though they were doing so makes such declarations moot.
> Their intent is faithfully record history, since the history itself persuades readers of Jesus' deity.
That is clearly not the case. The only Gospel to even look remotely like history is Luke. And he's a terrible historian, for the reasons I noted above. Now, I'll grant you that writing as though the stories actually happened was a very useful device for being convincing. That's why it was adopted, and why students were taught how to do it as standard practice in Greek literature of the time.
But I would note that this practice was clearly not undertaken from the beginning of the Church. Paul seems to have had no trouble preaching his gospel without any such stories, or indeed any reference to anything Jesus did on Earth at all. He actually had to defend himself to the Galatians against accusations that he had learned his gospel from a human, historical source rather than directly via revelation. So the earliest documents we have show us a church that was aggressively disinterested in history.