The find. The single most celebrated scroll is one of the seven from Cave 1: a nearly complete copy of the book of Isaiah, all sixty-six chapters on seventeen sheets of stitched leather about seven metres long, copied around 125 BC (some date it a little earlier). Known to scholars as 1QIsaa, it is by far the oldest complete book of the Hebrew Bible in existence, roughly a thousand years older than the Masoretic manuscripts that lie behind every printed Hebrew Bible before 1947.
What it showed (the fact). When scholars laid this second-century-BC Isaiah beside the medieval text, the overwhelming result was continuity: across a thousand years of hand-copying, it is recognizably, substantially the same book, the same chapters in the same order saying the same things, including the famous "suffering servant" of Isaiah 53. For a reader who wants to know whether Scripture drifted beyond recognition down the centuries, this is concrete, datable evidence that it did not. It is the traceable heart of the line "the word of our God stands forever" (Isaiah 40:8, Berean Standard Bible).
The honest counter-case (attributed, not minimized). "Substantially the same" is not "letter-for-letter identical," and careful scholarship says so. The Great Isaiah Scroll differs from the Masoretic text in thousands of small ways, mostly spelling and fuller vowel-letters (the way "favour" and "favor" differ), word order, and the occasional different or added word; a handful of readings are genuinely interesting to textual critics. What the differences do not touch is the substance: no teaching of Isaiah rises or falls on them. So the precise, honest claim, the one textual scholars such as Emanuel Tov actually make, is that the scroll demonstrates remarkably faithful transmission of a stable text over a millennium, not a miracle of flawless copying. That is a strong result stated truthfully; overstating it into "not one letter ever changed" is exactly what forfeits a careful reader's trust.