Why "Prove the Bible" is irrational

"Do you have proof that the Bible is factual?"
Can you tell me what is the Ultimate Standard for Truth? Who ultimately decides the difference between truth and fiction? As a Christian, I can answer this very easily: Truth and Fiction are defined by God, his word (the Bible) communicates this to us and mankind was created in God's image which means we understand the concepts ourselves. Unfortunately, every person also has a sinful nature which means that we naturally ignore these definitions and pretend that we know better. This also means that every one of us needs to accept the ultimate sacrifice Jesus made to save all of us from our sins.
Now throw this out the window and assume for a second that the Secular Humanism religion is the truth: On what basis does Truth exist in the first place? According to this system of beliefs, every single thing in the universe is nothing more than the result of a long sequence of chemicals randomly and accidentally reacting with each other. Thus, all thought is likewise nothing more than a bunch of random and accidental atomic collisions. On what basis are the random and accidental chemical reactions that someone calls "Truth" "better" than the equally random and equally accidental chemical reactions that someone calls "Fiction"? On what basis are the random and accidental chemical reactions that one person calls "Truth" "better" than the equally random and equally accidental chemical reactions that another person calls "Truth"?
These are very important questions because without an Ultimate Standard for Truth, one has no basis whatsoever in promoting their particular cherished beliefs as Truth or declaring as fiction anything that they disagree with.

C.S. Lewis summarised it pretty well:
"If Nature when fully known seems to teach us (that is, if the sciences teach us) that our own minds are chance arrangements of atoms, then there must have been some mistake; for if that were so, then the sciences themselves would be chance arrangements of atoms and we should have no reason for believing in them." (Lewis C.S., On Living in an Atomic Age, 1948)

Even Atheists have agreed with this conclusion:
"It seems to me immensely unlikely that mind is a mere byproduct of matter. If my mental processes are determined wholly by the motions of atoms in my brain, I have no reason to suppose my beliefs are true. They may be sound chemically, but that does not make them sound logically. And hence I have no reason for supposing my brain to be composed of atoms." (Haldane, J.B.S. Possible Worlds, p. 209)
"Why is thought, being a secretion of brain, more wonderful than gravity as a property of matter?" -Ernst Mayr’s book on Charles Darwin, One Long Argument: Charles Darwin and the Genesis of Evolutionary Thought, Harvard, 1991

Some secularists, in response, have tried to answer that the Ultimate Standard for Truth is whatever conforms to reality. However, this argument is flawed because reality meant different things to different people at different times. 100 years ago, continents fixed in place was reality. 300 years ago, Phlogiston theory was reality. 1000 years ago, Geocentricism was reality. Even today, reality is incomplete and changing. For example, in 2018, thousands of Americans were surveyed concerning the Flat Earth and the results showed that amongst the youngest age group (18-24), only 66% were convinced that the Earth is a sphere while older groups were progressively more likely to believe this ( https://www.forbes.com/sites/trevornace/2018/04/04/only-two-thirds-of-american-millennials-believe-the-earth-is-round/#632d02207ec6 ). This seems to indicate that the Flat Earth is slowly becoming the new reality. If you want to make this argument, my question is: Who decides what reality is and by what authority, keeping in mind the above? My counterargument though is that it is not correct to argue that reality defines the Ultimate Standard for Truth. Rather, the Ultimate Standard for Truth defines reality.
Another attempted answer is to say that evidence decides the Ultimate Standard for Truth whereas anything that contradicts the evidence is false. The problem is that this is an Reification Fallacy: Evidence is not a person, it does not "make decisions" or "say" anything. Rather, people look at the evidence and interpret it according to their worldview. All of those discredited ideas above were supported with evidence but look where they are now. The Flat and Spherical Earth sides both use evidence (in many cases, the same evidence) to "prove" their side and "refute" the other. If you want to make this argument, my question is: Who decides how evidence should be interpreted and by what authority, keeping in mind the above? My counterargument though is that evidence does not defines the Ultimate Standard for Truth. Rather, the Ultimate Standard for Truth defines how evidence should be interpreted.

Now, after reading all that, you're probably wondering: What does any of this have to do with my question? The answer is that the entire concept of "factuality" is meaningless without an Ultimate Standard for Truth.

Another problem with this line of reasoning is that it is a Double Standard because eyewitness testimony is generally accepted as factual unless there is a good reason to reject it, such as a contradiction with other testimony or forensic data. This applies to written eyewitness testimony as well. But when it comes to the Bible, suddenly people adopt the opposite attitude: The Bible is lying until proven innocent. Then, when one investigates why this attitude exists, one quickly finds that the Double Standard is not the only fallacy these proponents are pushing. For example, it is commonly argued that the Bible is invalidated because it makes supernatural claims. This is the fallacy of Begging the Question. Another common fallacy is the Argument from Silence, committed when secularists insist that a historical event in the Bible must be rejected because there is no evidence that this event occurred.

Finally, this stance is problematic in light of the numerous occasions where secularists declared a particular event in the Bible a myth only to have to eat their words once science caught up and demonstrated that the Bible was right all along. For example, King David was once declared a mythical person simply because the Bible provided the only evidence for his existence. But then archaeologists started finding artefacts that proved David a real person. Another example of this is Sargon II, King of Assyria. Also declared a mythical figure even though the Bible names him in Isaiah 20:1 and ultimately vindicated by Archaeologists discovering that the Bible was right all along. Of course, these embarrassments could have been avoided from the start if people had simply accepted the Bible's value as a historical document from the start. In fact, the Bible even enhances our knowledge of history from extra-biblical sources. For example, the assassination of King Sennacherib. If we only rely on Assyrian records, we know that Sennacherib was assassinated, a succession war broke out amongst his children and Esarhaddon became his successor. Said records are strangely silent on the identity of the assassins though. If we read the Bible, we find their identity: Adrammelech and Sharezer, two of his sons.

I hope this helps you understand that asking someone to prove the Bible is actually a pretty irrational question.