Is there any proof of Isaiah 53 predating Christ? I’ve heard carbon 14 testing for the Isaiah Scroll could be inaccurate as the parchment could be reused and paleographic dating might be unreliable.
There is a boat load of evidence for the Book of Isaiah predating Christ. Perhaps the most obvious is the fact that Jesus quoted Isaiah dozens of times in the existing four gospels. For example, Jesus quoted Isaiah 61:1-2 in a Synagogue service in Luke 4:19 and applied it to himself. This means that, not only did Isaiah exist as a book at the time, but that it had been around long enough to have become accepted as part of the Hebrew scripture by the Jewish teachers of the Law of his day. Even if we take the extremely skeptical position that Luke’s story is invented, we can say with absolute certainty that his readers in AD 65 or so, saw this as a credible story, so we know with 100% certainty that Isaiah was Jewish Scripture at the time of Jesus’ ministry. How long a time ensued between the time a prophetic book is written and it is accepted into the Jewish canon? Surely at least 100 years. So, this pushes Isaiah back to at least about 100 BC.
But this is just a beginning. Josephus tells us that Isaiah was part of the Hebrew canon, implying that it was Ezra who finalized that place for Isaiah. This would put the date of Isaiah into at least the fifth century BC, unless Josephus is mistaken. But Josephus is a generally reliable historian. Since he lived in the first century, it is not likely he would be completely mistaken about the age of Isaiah, so this is evidence (not proof) of Isaiah being at least four hundred years older than Christ.
Then there is the Dead Sea Scroll evidence. The 1Q Isaiah scroll was tested in Tucson in 1994 (Doudna, G. “Carbon-14 Dating”, in Encyclopedia of the Dead Sea Scrolls, Schiffman, Lawrence, & VanderKam, James, eds., Vol.1 (Oxford: 2000)) and produced a date of 142 +/- 90 BC. Sure, these numbers have an uncertainty, but the evidence puts this scroll well before Christ, even if we take into account the uncertainty. As for the parchment being reused, sure, but the Jews never put their sacred Scripture on reused parchment!!! You can reject that criticism of the date of the Isaiah scroll for sure. No, this scroll is from roughly 140 BC. Also, the two most important Isaiah scrolls are roughly 100 BC and 150 BC, based on “paleographic” evidence. How reliable is such evidence? You say that you have heard this might be unreliable. Well, sure, but the reliability is a relative thing. For example, if the paleographic experts say that Isaiah A was from 150 BC, perhaps that date is unreliable, but how unreliable? Let us concede that it may indeed only be from 100 BC. There may be a bit of doubt for this data, but the amount of doubt certainly does not place it at AD 50. So, let us say that the paleographic data says at least 100 BC to be conservative. This means that Isaiah was already in the Hebrew canon by 100 BC, which means that surely it was written before 200 BC, allowing for the time between the writing of a prophetic work and its general acceptance into the canon. (I am assuming here that the Qumran community were using Isaiah as Scripture, which I believe is quite certain).
Then there is the evidence that the Hebrew Scripture were translated into Greek around 230 BC in the translation known as the Septuagint. This conclusion is a bit tentative, but scholars generally will agree that Isaiah was translated into Greek, as part of the Hebrew Scripture, certainly before 200 BC, which puts its date of writing somewhere before 300 BC.
Also, there is the fact that Jewish teachers were using Isaiah at a verifiably early date. To be honest, I have not done much research here, but the use of Isaiah in the Targums, the Mishna and the Talmud will put Isaiah at least as old as 200 BC, but almost certainly considerably older than that.
My conclusion is that what we call the Book of Isaiah EXTREMELY likely predates 300 BC, but any attempt to put it after 200 BC is proved wrong beyond a reasonable doubt by all the data. You asked about proof. I always prefer to use the word evidence rather than proof, but in this case the word proof is appropriate. Yes, it is proved beyond the shadow of a doubt that Isaiah 53 predates Christ.