Why Gospel Authors Would Have Been Named Early

- Christianity wasn't a philosophical system of ideas that were being promoted independently of authority figures. Rather, it was a system founded on the authority of named individuals, starting with Jesus and going on to the apostles and other individuals who were named (Matthew 10:1-3, Mark 3:13-19, Ephesians 2:20, etc.).

- Luke's gospel opens with a reference to the significance of eyewitnesses (1:2), a concept that requires distinguishing among sources (differentiating between those who were eyewitnesses and those who weren't), which would include distinguishing among the authors of written sources. While authors could be distinguished without naming them, the use of names is more efficient and more common. The burden of proof rests on those who maintain that the gospel authors were distinguished without being named early on.

- Luke refers to his use of prior sources in the opening of his gospel, and the written nature of his own work makes it unlikely that he's referring only to oral sources. For more on that subject, see here. There's widespread agreement, across the scholarly spectrum and among non-scholars, that Luke at least used one of the other canonical gospels as one of his sources. And once two or more gospels of such prominence were in use, there would be a need to distinguish among them in libraries, when using them during church services, and so on. We have a lot of evidence that the gospels were distinguished in such contexts by means of authorship attributions from the second century onward. And continuity is more likely than discontinuity. It makes more sense that the gospels were distinguished by means of author names in the first century than that they weren't. That scenario better explains the widespread acceptance of the practice later and the absence of any comparable or better alternative.

- The fourth gospel expresses an interest in its own authorship, and that carries with it the implication of concern about authorship more widely (John 21:24). For more about the passage just cited, see here.

- There are passages in the other three gospels and Acts that raise interest in the authorship of those documents. That's especially true of Luke and Acts. The references to Theophilus and the "we" passages beginning in Acts 16:10 would have led ancient readers, not just modern ones, to think about who was doing the writing. Who was Theophilus, and who would be writing to him under the circumstances in question? Who would be writing from the perspective of "we" in the relevant passages?

- Ancient literary practices and the manuscript record suggest that the gospels circulated with the authors' names attached from early on. Tertullian went as far as to say, "here I might now make a stand, and contend that a work ought not to be recognised, which holds not its head erect, which exhibits no consistency, which gives no promise of credibility from the fulness of its title and the just profession of its author" (Against Marcion, 4:2).

- Papias, who lived during the second half of the first century and the first half of the second, was interested in gospel authorship, and he cites an earlier source, referred to as "the elder", who was interested in the subject (in Eusebius, Church History, 3:39:15).

- The early heretics were concerned about authority figures and authorship attributions, as we see in the dispute between Marcionism and mainstream Christianity, for example. The early heretics often drew material from the gospels, commented on them, produced their own variations of them, etc. See, for example, C.E. Hill's discussion of early heretical sources in Who Chose The Gospels? (New York: Oxford University Press, 2010). For a discussion of how heretics of the early second century onward possessed and commented on the New Testament documents, including the gospels, see Bruce Metzger, The Canon Of The New Testament (New York: Oxford University Press, 1997), 75-106.

- The early Christians often distributed the gospels to non-Christians in general, including Jewish and Gentile non-Christians who weren't heretics (John 20:31; Quadratus, as discussed in Eusebius, Church History, 3:37; Justin Martyr, Dialogue With Trypho, 10; Aristides, Apology, 2; Theophilus of Antioch, To Autolycus, 3:15; Tertullian, Apology, 31). Near the beginning of Justin's Dialogue With Trypho, his Jewish opponent mentions that he's read the gospels (10). Celsus and his Jewish source(s) are familiar with the gospels as well and frequently refer to them in one way or another. In the abstract, we'd expect the opponents of Christianity in the earliest centuries to be interested in gospel authorship, and, in fact, they do often describe or name the authors of the gospels.

- The large number and variety of evidences for early authorship attributions for the gospels are accompanied by an absence of references to anonymous gospels. That includes sources who had a lot of interest in mentioning that the gospels had initially circulated anonymously if they had.