Since the skeptic has what Leolaia refers to as "a sustained skepticism" against "a supernatural basis" the skeptic needs no real evidence to assert that every biblical prophecy or revelation is not credible.
And what about you? Do you intuitively accept claims concerning UFOs, telepathy, psychic surgery, and other supernatural phenomena at face value, or do you want to see evidence to substantiate such phenomena? Are you equally credulous of all prophecies and revelations that appear to have had fulfillment, without asking yourself if the "prophecy" was written later? Do you accept on its face the pre-Columbian revelation made in the Book of Mormon almost two thousand years ago that predicted Columbus, the American colonies, the Revolutionary War, and even the discovery of the Book of Mormon itself in 1823 with astonishing accuracy? I would guess you would be very critical of those who want to see evidence that the Book of Mormon is really a pre-Columbian document. How about the Potter's Oracle? It claims that it was written in the 18th Dynasty of Egypt under one of the Amenhoteps, and yet it foretells events that would not happen until the Seleucid era over 1,200 years later! Egyptologists agree that this prophecy was really written around 170 BC, but I am sure you would agree that this dating is simply due to an anti-supernaturalist bias.
Therefore, since it is always possible for the skeptic to claim (claim but not prove) that every Bible prophecy was written after-the-fact, and the skeptic can always assert that there is no such thing as the supernatural, the skeptic has created a perfect firewall around their skepticism.
There's a huge sweeping exaggeration. No informed skeptic would claim that every biblical prophecy was written after the fact; it is patently clear that many if not most make claims regarding the future from the standpoint of their authors. I would say that ex eventu prophecies are definitely in the minority in the Bible. Much of what we find in Ezekiel, Jeremiah, Revelation, etc. is generally accepted as oracles made at the putative authors at the time of their supposed composition. Whether all these prophecies ended up being fulfilled is quite another question. And we still have to recognize that the wording may change through later redaction and copying of the prophecy, as we can quite strikingly see in the case of Jeremiah where "improvements" have been made to the text along the way. You do not seem to admit that this phenomenon should be taken into consideration.
What you regard as a "firewall" is only a very high burden of proof. It can be met. One could say that skeptics have a similar "firewall" about the existence of UFOs. But if a mothership hovers above every major city and aliens begin interacting with the populace as seen in the TV program "V", then that would settle any reasonable doubt and constitute the "proof" needed to establish the matter. A hunter could bring a carcass of Bigfoot to a team of doctors, zoologists, and geneticists and thereby establish the existence of Bigfoot as a "fact". Similarly, if everyone on earth sees a glorious figure coming on the clouds with a loud voice and if the dead then are seen to arise from the graves, that would similarly prove the realiability of the parousia prophecy as a "fact". Or if the book of Revelation that we've had for thousands of years contained a list of names that later turned out to be names of US Presidents and which could be used to predict the outcome of future elections, that would similarly more than satisfy the burden of proof. Most prophecies, however, don't work that way; they more commonly tend to be vague and open to many different interpretations.
It becomes irrelevant to the skeptic that no copy of Isaiah exists that does not contain the prophecy written in 7:14 or 51:14. Yet, the skeptic claims that Isaiah was first written without such prophecies, that they were all added "later," after the prophecies were fulfilled. But no such copy without them was ever discovered
You do realize that the earliest copy of Isaiah at Qumran dates to c. 100 BC, over 600 years after the time the prophet lived? There are limits to textual criticism. Let's similarly eliminate the first 600 years of textual transmission of the NT, such that the earliest manuscripts date to the seventh and eighth centuries AD. I guarantee you that there would be interpolations and scribal changes to the text of the NT that would no longer be detectable. Or consider the case of the Quran. When Uthman standardized the text some 20 years after the death of Mohammad, his recension became the single canonical text and he had all earlier copies destroyed. These copies contained textual variants that would have illuminated how the text developed. But with their destruction, there would only be a single authortative version; it is impossible to use textual criticism to trace the development of the text since all existing copies are derived from Uthman's edition which is still extant. Textual criticism is not the only tool for investigating the history of the text; literary analysis considers linguistic, redactional, and stylistic evidence and extratextual evidence also helps establish a text's provenance and date. The evidence indicating that Deutero-Isaiah did not originally belong to the text of Isaiah far outweighs the probative value of extant manuscript evidence which is far too late to exclude the probable date of composition of Deutero-Isaiah. What we need is a manuscript of Isaiah dating to the seventh century BC that does contain ch. 40-66. That is the kind of textual evidence that would establish the early date of this material, in contrast to all the other signs pointing to a later date.
Similarly, no copy of Daniel exists without chapter 8.
And yet your copy of Daniel (unless you are reading a JB or similar version) somehow lacks the two psalms in ch. 3 and the two stories in ch. 13-14. The LXX also has entirely different versions of the stories in ch. 4-6. Some manuscripts completely reshuffle the order of the chapters. All of this shows that the text of Daniel was pluriform in antiquity. And the structure of the book contains many features that indicate that the book is a composite (with an older Aramaic apocalypse in ch. 2-7 augmented by a later Hebrew apocalypse in ch. 8-12 and ch. 1), the evidence pertaining to which you may readily examine in the academic literature devoted to the composition of Daniel.
It becomes disingenuous after a while to hear that the book of Daniel was "altered" in 300 BC (after Alexander's empire was split up) to add-in after-the-fact prophecies about Alexander, even though the book had been read, memorized, and studied for 200 years by the people who had been in exile and brought the book back with them from Babylon. They and their children knew the book inside and out and would have known it had been modified by scribes who were not even prophets.
I could similarly say that it is disingenuous to read that the book of 1 Enoch was written "after the fact" to add in prophecies pertaining to the second century BC, even though it had been read, studied, and appreciated for thousands of years before then. My evidence that the book of 1 Enoch existed prior to the time when literary criticism finds it was written? About the same as the evidence from the Persian era that Daniel was read and studied and memorized during that time. There is no trace of Daniel in any of the literature of the Yehud until it suddenly pops up in the second century BC and thenceforth its influence is felt all over the place. Heck, even the book of Daniel itself claims (in 11:35, 40, 12:4, 9) that it was "sealed up" in the centuries intervening the time of its purported composition (in the sixth century BC) and its subsequent publication and release during the Maccabean crisis described in ch. 11. If the book was still "sealed", it was not being read. This is of course a literary device to explain why no one had seen this book before. The exact same literary device occurs in other books like the Assumption of Moses (which is used as a source in the epistle of Jude). John of Patmos reversed it in Revelation 22:10 ("Do not seal up the words of this book, for the time is near") precisely because his book was intended to be read and circulated immediately and not sealed up for a later generation as Daniel had supposedly been.
Remember, Israel considered themselves to have had NO prophets from 400 BC until Christ was born.
Which is precisely why there are books like Daniel, 1 Enoch, the Assumption of Moses, the Testament of Levi, etc., as writers with a prophetic message during this period had little choice but attribute their visions and oracles to figures of old in order to gain an audience. It was right in that "intertestamental period" when apocalypses flourished as a literary genre, books which made their impression on the ideas and literary form of the NT.
They and their children knew the book inside and out and would have known it had been modified by scribes who were not even prophets. How remarkable if the skeptic is right! No one noticed the changes to the sacred writings, and all the true original scrolls were all lost just before being preserved at Qumran? That is hard to believe.
And yet we know that changes did occur because we see them right in the versions of the "sacred writings" found at Qumran, in the LXX, and in other quarters. I think you seriously underestimate the sheer amount of textual diversity and tolerance of it throughout the period. You seem to think that it is incredible to think that such redactions and changes occurred to the text, yet it did because the evidence is there for all to see. On what basis do you claim that "no one" noticed that there were changes and variants? Certainly not the scribes of Qumran who often tried to correct copies on the basis of what other copies say (that is how additions commonly creep into the text, whether from marginal notes or when a scribe thinks he is restoring something he thinks was accidentally deleted but preserved in another copy). And certainly not the revisers of the Old Greek (such as those behind the Proto-Theodotionic kaige recension) who clearly were interested in correcting a text to make it conform to another.
Since the books exist as they do, repleat with prophecies, the believer does not have to explain them away. The burden of proof really is on the skeptic to "prove" that what exists were all faked.
Right, so I take it that you similarly accept at face value the amazingly accurate prophecies in the Potter's Oracle, the Dynastic Prophecy, the Animal Apocalypse of 1 Enoch, the Assumption of Moses, the Book of Mormon, and whatever else I can think of that "skeptics" consider to be ex eventu prophecies. Or do you choose not to be a "believer" in those prophecies? But if you doubt that they are not genuine prophecies, you are a "skeptic" like those you criticize who are not "believers" of the prophecies that you do accept. What is the more extraordinary claim — that some people may write books after the fact to give legitimacy of their own predictions of the future or that some people have supernatural powers to foretell the distant future? I have trouble seeing the former as more extraordinary than the latter. That is not to say that the latter is not possible, but only to say that the remarkable claim inherently has a much higher burden of proof than the alternative. Otherwise you are simply making an Appeal to Ignorance fallacy. As I already said above, you are perfectly entitled to accept such prophecies on faith and simply believe regardless of what the evidence might say. But a critical inquiry is going to treat all self-styled prophecies the same way, whether biblical or not, all are expected to meet the same burden of proof. There is no reason that those within the biblical canon must be accepted by literary critics at face value (on faith, as a "believer" would) until proven otherwise while a wholly different approach is taken with respect to prophecies outside of this canon.