Modern translations of the Hebrew Bible are largely based on a single manuscript called Codex Leningradensis, dating to the year 1000 C.E. The text of the Hebrew Bible was preserved and protected through the work of the Masoretes (500 – 1000 C.E.), who fixed the consonantal text and added to it vowel points for ease of reading and interpretation. The Dead Sea Scrolls show that many of the books of the Tanakh were copied carefully by scribes from the beginning of the common era; other books were changed to a greater extent, however, and it is impossible to know how much they were changed before the common era.
The Greek New Testament is contained in many more manuscripts – over 5,600 Greek manuscripts of all or part of the New Testament altogether. These thousands of manuscripts contain hundreds of thousands of differences, or variant readings. Even though the vast majority of these variants are highly insignificant, some of them do matter a great deal for interpretation and meaning; and there are numerous places where scholars have not been able to agree as to what the original text actually said.
The Bible: A Historical and Literary Introduction by renowned biblical scholar and New York Times bestselling author Dr Bart D. Ehrman, p. 406.