Ehrman goes on to discuss the criteria with which the early church used to decide what was to be "canonised". The quote by McDonald summarises, with references, essentially what Ehrman says about this process.
The generally accepted criteria for canonicity, however unevenly applied by the early church, included: (1) apostolicity, that is, whether a writing was written by an apostle (Eusebius Hist. Eccl. 3.25.4–7); (2) orthodoxy, whether the writing conformed to a widely accepted canon of faith, or regula fidei (Eusebius Hist. Eccl. 3.25.6–7 and also Serapion’s criteria for his rejection of the use and reading of the Gospel of Peter in his churches [c. 195] discussed in Eusebius Hist. Eccl. 6.12.1–6) (3) antiquity, whether it was written during the apostolic period; and (4) usage, whether it was generally accepted in prominent or large numbers of churches and used in their worship and catechetical programs (Eusebius Hist. Eccl. 3.24.18 and 3.25.6).
It appears at this point that Ehrman then leads himself into a bit of an historical conundrum. He argues, ala much NT scholarship, that the "authors" were manufactured and later added to the writings of the NT. This is especially made with reference to the gospels. But why would a document gain such a wide hearing, and authority if the author could not - at least to some degree - be verified or traced? Paul's letters were widely circulated beyond the churches that he specifically wrote to [Col 4:16; 2 Pet 3:16]. And with regards to the Gospels, Hengel's arguments [Four Gospels and the One Gospel of Jesus Christ] have been neglected by Ehrman, thus weakening his position considerably.  L. M. McDonald, “Canon” in DLNTD