Before the rise of the printing press, documents were produced by copyists ("scribes"). Inevitably, in spite of scribal care and skill,1 variations from the parent text crept into the manuscript tradition. In the absence of any original copies, the task of "texual criticism" is to discern the most original form of the text, given the available manuscript evidence.
The text histories of the hebrew-bible and the (Greek) new-testament are quite different, leading to slightly different operations and goals between text critics of Tanakh and New Testament, although the principles by which they operate remain the same.
Until the discovery of the dead-sea-scrolls in 1947, the oldest manuscripts of the Hebrew Bible were the great medieval codices of the Masoretic tradition, although these date from about 1000 CE. Because of this, the septuagint -- the pre-Chrstian Greek translation of the Hebrew scriptures for Jews of the Diaspora -- has long been recognized as an important source of information of the form of the text from the time of its production, with an obvious and additional layer of complexity because it is a translation.
With the New Testament, however, there is a continuous manuscript history from about the 3rd Century, and very many (thousands) of copies of manuscripts.
- Bruce K. Waltke, "Aims of OT Textual Criticism", Westminster Theological Journal 51.1 (1989): 93-108.
- TC: A Journal of Biblical Textual Criticism - open access.
- NT Gateway - Textual Criticism Resource Pages.
- "Essential Works in New Testament Textual Criticism".
1 See in general, Alan R. Millard, "In Praise of Ancient Scribes", The Biblical Archaeologist 45.3 (1982): 143-153.