Prior to the 1940 – 1950 period, the earliest copies of the Old Testament were from around 900 A.D. The Dead Sea Scrolls were discovered in the late 1940’s, but weren’t really investigated till the mid-1950’s, and the scholarly work continued for years.
One of the greatest discoveries in the Dead Sea Scrolls was a complete copy of Isaiah, one of the largest books in the Old Testament. Called “The Great Isaiah Scroll” because of it’s large size, scholars dated it to about 150 B.C.
So now we had a test. We had a copy of an Old Testament book which was dated a full 1000 years prior to the copies that had been used for many centuries. The scholars could compare the two versions and determine how accurately they were copied during the 1000 year period. Keep in mind that this was from 100 B.C. to 900 A.D., and all that was used was manual copying. The books were copied by hand for 1000 years. How accurate could the ancients be?
The book A General Introduction To The Bible by Geisler and Nix presents a good examination of the similarities and differences. They say the following:
Of the 166 Hebrew words in Isaiah 53 only seventeen Hebrew letters in QIsb differ from the Masoretic Text. Ten letters are a matter of spelling, four are stylistic changes, and the other three compose the word for “light” (add in v.11), which does not affect the meaning greatly. Furthermore that word is also found in that verse in the LXX and QIsa.
(Geisler and Nix, 382)
Keep in mind this was over a 1000 year period. By comparison, we have copies of the New Testament books far earlier. The earliest fragment of the New Testament is the John Rylands fragment, dated to between 20 and 50 years from the original. If scribes can copy the Old Testament accurately for 1000 years, they can copy the New Testament over one generation.
Therefore we can rest assured that the Bible has been copied accurately since it was revealed to us by God.