Here is a great article by Michael Kruger at his site:
By Michael Kruger
One of the standard challenges for New Testament textual criticism is whether we can work our way back to the original text. Some scholars are notoriously skeptical in this regard. Since we only have later copies, it is argued, we cannot be sure that the text was not substantially changed in the time period that pre-dates those copies.
Helmut Koester and Bart Ehrman are examples of this skeptical approach. Koester has argued that the text of the New Testament in the earliest stages was notoriously unstable. Most major changes, he argues, would have taken place in the first couple centuries.
Ehrman makes a similar case. Since we don’t have the originals, and only copies of copies of copies, then who knows what the text was really like before our extant copies were made.