The Bible itself has always claimed to be "written by humans", moved by God according to his interactions in history, speaking his words according to the authority given them.
The Bible itself has also been against false prophets (those claiming to speak for God), insisting that every word should be tested according to what came to pass.
For example, take the book of Isaiah. It claims for itself to be written by a human, Isaiah the son of Amoz during the reigns of Uzziah, Jotham, Ahaz and Hezekiah, kings of Judah. We know now it was written circa 700 years before Christ. Isaiah speaks about the suffering and rising of the Christ (see Isaiah 53 for an accessible example).
Now the question is, does that not match what we know of Jesus of Nazareth, and do the words of Isaiah not also authenticate Jesus of Nazareth?
And when Isaiah speaks about Cyrus the Great of Persia, many years before his time, does that not match what we know of Cyrus? And was Cyrus himself (according to first-century Jewish Roman historian Josephus) not amazed when he read Isaiah's words concerning himself? Or should we force the book of Isaiah to be post-dated so as to sit more comfortably with a naturalist philosophy?
According to textual criticism (the branch of research where you apply source criticism to the biblical texts the same way you would for any other historical source), the book called Isiah is a compilation of texts written by three different authors at different points in time - before, during and after the exile.
The prophecy in Isiah 53 talks about a guy who was incredibly ugly and despised by everyone, who was killed disgracefully, and who never uttered a word. This probably matches numerous unfortunate souls before and after, but clearly not Jesus, since he was actually revered by his followers and did utter quite a lot of words, even during his execution. In other words, the passage disproves that Jesus could be the Messiah - unless you apply a healthy dose of cognitive dissonance.
You reduce the passage to being about "a guy who was incredibly ugly" whereas the context (see Isaiah 52) indicates someone beaten horrifically, "his appearance was so disfigured beyond that of any human being and his form marred beyond human likeness".
Your caricature neglects to mention and skirts around the key details of the passage:
The suffering servant is the kingly representative of his people, appointed by God, who, though innocent, will bear the punishment of his people's sins on their behalf. He will be smitten by God and they will mock him for it. He will be rejected, suffer and die, but he will rise, and make many to be accounted righteous.
The substitutionary atonement is clearly repeated many times in Isaiah 53:
"Surely he took up our pain and bore our suffering, yet we considered him punished by God, stricken by him, and afflicted."
"But he was pierced for our transgressions, he was crushed for our iniquities; the punishment that brought us peace was on him, and by his wounds we are healed."
"We all, like sheep, have gone astray, each of us has turned to our own way; and the Lord has laid on him the iniquity of us all."
"For he was cut off from the land of the living; for the transgression of my people he was punished."
"He was assigned a grave with the wicked, and with the rich in his death, though he had done no violence, nor was any deceit in his mouth."
"Yet it was the Lord’s will to crush him and cause him to suffer, and though the Lord makes his life an offering for sin, he will see his offspring and prolong his days, and the will of the Lord will prosper in his hand."
"After he has suffered, he will see the light of life and be satisfied; by his knowledge my righteous servant will justify many, and he will bear their iniquities."
"For he bore the sin of many, and made intercession for the transgressors."
If we extract the "factual" statements about the figure in the prophecy, you will see it does not match what we know about Jesus. All the matches are theological construction after the fact to make the pieces fit. And you have to ignore or gloss over the parts that does not fit: "He was oppressed and afflicted, yet he did not open his mouth".
No, I was just quoting from Isaiah there. That's Isaiah speaking many years before Jesus, describing and requiring the Christ to be a Suffering Servant.
Not many ancient historians would contest that Jesus was a historical person, or have any reason to question that he died the way he did. Nor would they contest the public life of John the Baptist or Pilate or Herod for that matter, or that the New Testament documents contain eye witness testimony, as well as incidental historical detail in the form of taken-for-granted shared understanding of historical events in the minds of those who were then alive and able to contest and examine the facts. Many of those who witnessed to Jesus suffered for their convictions, and faced tremendous pressure and incentive to quit.
"And you have to ignore or gloss over the parts that does not fit: 'He was oppressed and afflicted, yet he did not open his mouth.'"
Have you read the remarkable account of Jesus before Pilate?
At the end of the day, you must decide for yourself.
You are correct most historians assume a historical Jesus did exist. Just as they assume Mohammad and Gautama Buddha existed.
With respect, "God" is just a term referring to a being that has supreme powers that humans imagine to be the best of the best - and different people define these powers and other attributes differently. So even if you are an atheist, the study of Gods can still be an interesting reflection of people and society!