Bart D. Ehrhman, The New Testament: A Historical Introduction to the Early Christian Writings, sixth edition (OUP, 2016), 25.
In the earliest centuries, the vast majority of copyists of the New Testament books were not trained scribes. We know this because we can examine their copies and evaluate the quality of their handwriting, and we can assess how accurately they did their work. The striking and disappointing fact is that our earliest manuscripts of the New Testament have far more mistakes and differences in them than our later ones. The earlier we go in the history of copying these texts, the less skilled and attentive the scribes appear to have been.
The Amateur Exegete 