When we scrutinise the Gospel authors in the light of their identities and content and date of their writings, we will find that they are not credible eyewitnesses to the crucifixion. To begin with, it's important to recognise that the Gospels themselves are, strictly speaking, anonymous. While today in the New Testament you see the headings "The Gospel according to..." at the start of each of the Gospels, it's important to note that none of the authors identify themselves by name within the texts. They were quoted anonymously by Church Fathers in the first half of the second century (i.e. 100-150 CE) and the names by which they are currently known appeared suddenly around the year 180 CE, nearly 150 years after Jesus. We find this in the writings of early church apologists such as Justin Martyr who was writing in the middle of the second century. Justin quotes from the gospels on numerous occasions, but the striking ting is that he does not call the Gospels by their names. Instead, he regularly calls them "Memoirs of the Apostles." He does not say that he thinks the disciples themselves wrote the books, only that these books preserve their "memoirs" (meaning, their recollections of the life and teachings of Jesus). These are some of the reasons that have led scholars to believe that the names Matthew, Mark, Luke and John were assigned to the Gospels long after they were first authored.Given that we do not possess the original autographs for any of the four gospels (but only copies of copies), how can Zakariya state so confidently that the gospels were originally published anonymously? In fact, every extant Greek manuscript we have of one of the gospels that possesses a front page lists the author to whom it has been traditionally attributed. True, front pages tend to be rare the earlier you go back (the beginning and end of books tend to take the most damage), meaning that the majority we have are later -- but my point is, how can we be so sure that the original gospel autographs were anonymous, given that we do not have access to them? Moreover, Luke must surely have been known by Theophilus to whom he addressed both his gospel and his Acts of the Apostles.
Furthermore, during the time of the papyrus scroll, the name of the author would often not be mentioned in the text. Herodotus and Thucydides did mention their names in their respective texts, whereas Suetonius and Plutarch did not. Even Josephus doesn't state his name in the text of Antiquities of the Jews. Names were often placed not in the text itself but rather at the end of the manuscript.
Even the first century church father, Clement of Rome, does not mention his name in the text of his still-extant epistle to the Corinthians. Rather, we know who wrote it from the testimony of Irenaeus of Lyons. Irenaeus of Lyons, by the way, is an important witness in regards to the authorship of the gospels, for he was himself (by his own confession) a disciple of Polycarp of Smyrna who was himself, Irenaeus tells us, a disciple of John the apostle. This makes Irenaeus remarkably close to the apostolic generation, and boosts his credibility as someone likely in a position to know the true authorship of the gospels -- especially for John's gospel which he attributes to John the apostle.
The authorship of Matthew is attested by Clement of Alexandria, Irenaeus of Lyons, and Papias of Hierapolis. The authorship of Mark is attested by Tertullian of Carthage, Irenaeus of Lyons, Clement of Alexandria, and Papias of Hierapolis (incidentally, all of these witnesses also attest that Mark based his gospel on the eyewitness testimony of the apostle Peter). The authorship of Luke is attested by Clement of Alexandria, Tertullian of Carthage, and Irenaeus of Lyons.The authorship of John is attested by Tertullian of Carthage, Irenaeus of Lyons and Clement of Alexandria. Moreover, the authorship of all four gospels is attested by the Muratorian fragment, the earliest canonical list of the New Testament books (dating to around 170 C.E.).
Papias of Hierapolis, by the way, who attests to the traditional authorship of Matthew and Mark, wrote around 125 C.E. So Zakariya is not entirely correct that there are no attributions of authorship in the first half of the second century. We don't have Papias' original work but what we do have is preserved in quotations from the fourth century church historian Eusebius of Caesarea. Nonetheless, Papias is an early second century witness. According to Irenaeus, he was "an ancient man who was a hearer of John and a companion of Polycarp." In fact, Papias himself tells us (in a quotation preserved by Eusebius -- Hist. Eccl. 3.39.3–4):
I shall not hesitate also to put into ordered form for you, along with the interpretations, everything I learned carefully in the past from the elders and noted down carefully, for the truth of which I vouch. For unlike most people I took no pleasure in those who told many different stories, but only in those who taught the truth. Nor did I take pleasure in those who reported their memory of someone else’s commandments, but only in those who reported their memory of the commandments given by the Lord to the faith and proceeding from the Truth itself. And if by chance anyone who had been in attendance on the elders arrived, I made inquiries about the words of the elders—what Andrew or Peter had said, or Philip or Thomas or James or John or Matthew or any other of the Lord’s disciples, and whatever Aristion and John the Elder, the Lord’s disciples, were saying. For I did not think that information from the books would profit me as much as information from a living and surviving voice.If Papias really was connected to John and/or other apostles, and a companion of Polycarp, then he was in a key position to know the authorship of the gospels.
Furthermore, consider the geographical spread of the attestation of gospel authorship, as represented by the figure below: