We have over 30,000 copies of books from before they decided what would be included. Their decision on what would be included was mainly based on prevalence and wide distribution of a candidate, the authorship, and uniformity of doctrine. Obscure "gospels" like Thomas and many many others were left out because they had just a few copies, from a small area with suspiciously aberrant doctrines. The lack of wide distribution makes it most likely that it is not a old copy of a text, because of the traditions associated with copying and distributing everything they could get their hands on. When you take the 30,000+ texts and compare them, you find no deviations aside from occasional misspellings, punctuation differences and physical damage. Across all of the deviations, not a single doctrine is brought into question.