The question is, “In the history of the passing down of the Gospel traditions, how do we know that the later scribes or copiers of these manuscripts did not ‘over-egg the pudding’ or did not doctor the evidence to make it look more Christological and more Trinitarian, etc.?” This is a perfectly fair question based on the study of the some 5,000 pieces and whole manuscripts that we have of the Greek New Testament.
Firstly, the scholars who say this, including Dr. Ehrman, are right that there are no two manuscripts that are identical. Secondly, there were some tendentious scribes. There were some scribes out there who felt that they could play fast and loose with the material, and change it to suit their particular theological agendas. There is such a thing as the doctoring of the evidence by some scribes. That is one of the purposes for text criticism. We are trying to get back to the original texts – ad fontes, ‘back to the origins’. What was it that Matthew, Mark, Luke, and John actually said, and can we evaluate it? That is the point of text criticism – to get back to the original state of the document. So yes, there is normal editing, and there is tendentious editing. There is normal copying, and there is tendentious copying. What there is not, so far as I can tell, is some kind of conspiracy to cover-up what the original texts said.
A really good example is the western text of Acts versus the Alexandrian text of Acts. The western text of Acts has 20% more material in it than the Alexandrian text of Acts. It is called the western text because it probably came from the western end of the Empire. The Alexandrian text was a text copied in Alexandrian in Egypt. So what is the difference here? Well the western text has value added. It heightens the role of Peter; it pumps up the story of the baptism of the Ethiopian eunuch; it adds the word “Holy” to the word “Spirit” about 15 times. It is clear that the scribe who produced the original copy of the western text of Acts wanted an amplified version. He is not so much distorting things, as clarifying it. In a world where there were no footnotes, he was putting the clarification right in the text. We know that that happened. But we have all kinds of criteria by which we can evaluate which is more likely to be the original reading. Which is the more difficult reading? Which is the reading that best explains all of these later variants?
The bottom line is I do not see any evidence of a conspiracy theory – that is an attempt to cover-up original texts. I see plenty of evidence of attempts to amplify, clarify and rephrase what the original texts said in ways that are helpful to the 2nd and 3rd Century church. Some of this is just pastoral. They are assuming that the literate persons who would read this or hear this might not would understand it without clarification. Because the church of the 2nd and 3rd Centuries is very different from the very largely Jewish church of the 1st Century, the readers and listeners needed some help.