The authors of the Gospels were eyewitnesses, or close contacts of eyewitnesses

Summary:

How can we be sure that the four Gospels were written by Matthew, Mark, Luke and John? By studying what the earliest historical sources tell us about these texts. If the Gospels had initially circulated anonymously, it is certain that we would now have a multiplicity of attributions regarding their authorship. But this is not the case. Tradition has always held that Matthew's Gospel was written by Matthew; Mark's by Mark (Peter's interpreter); Luke's by Luke, who was a physician (Col 4:14) and Paul's companion; and John's by John, the son of Zebedee.

Historical sources

These facts are confirmed by six primary historical sources, each from different authors, scattered throughout the world. One literary work's author being attested and referenced by multiple sources is a rare occurence for that time period: the common rule is that some works' authors are attested by just one source. As an example, if we take Tacitus' famous Annals, we find that only Saint Jerome asserts that Tacitus is its author, and this a whole 300 years after the publication of the original version! On the other hand, here is the list of the six oldest sources attesting to the traditional authorships of the Gospels:

Papias, bishop of Hierapolis (around 125)

Around the year 125, Papias, a bishop of Hierapolis (in today's Turkey) who knew Saint John, declared (as reported by Eusebius of Caesarea): "Matthew therefore wrote the oracles in Hebrew, and everyone interpreted them as they could. Mark, who had become Peter's interpreter, wrote down accurately, but in no particular order, all that he remembered of the things said or done by Christ. In fact, he did not hear the Lord and did not follow him, but afterwards, as I have said, he followed Peter, who adapted his teaching to the needs of his hearers, but without intending to give a coherent account of the Lord's discourses, so that Mark did not make mistakes in writing down certain things as he remembered them. Indeed, he was careful not to omit anything he had heard and not to report any falsehoods" (Eusebius of Caesarea, Ecclesiastical History, Book III, 39).

Justin Martyr (around 150)

Around the year 150, Saint Justin Martyr called the Gospels the "Apostles's memoirs" and certified that they had been written by them: "Indeed, the apostles, in the memoirs they composed and which are called Gospels, have transmitted to us what was prescribed to them" (First Apology, 66).

Muratori fragment (circa 170 AD)

The Muratori fragment, dated by most experts - Christian or otherwise - to around 170 AD, tells us that Luke and John were indeed the authors of the Gospels. The beginning of the fragment is missing, and it ends abruptly. But as, after an incomplete sentence, it speaks of the third Gospel as being that of Luke, and then of the fourth Gospel as being that of John, historians admit that the missing part at the beginning of the paragraph must have mentioned the Gospels of Matthew and Mark:

"[...] The third book of the Gospel is that of Luke. Luke, the well-known physician, after Christ's ascension, and when Paul had taken him with him as a zealot for the law, composed it in his own name, according to [general] belief. However, he had not seen the Lord in the flesh, and so he began to tell the story from the birth of John, since he was in a position to establish these events. The fourth Gospel is that of John, one of the disciples".

Irenaeus of Lyon (around 180)

In the year 180, Saint Irenaeus, a disciple of Polycarp, himself a disciple of Saint John, gave a precise account of the origin of the four Gospels as a whole: "Matthew published for the Hebrews, in their own language, a written form of the Gospel, at the time when Peter and Paul were evangelising Rome and establishing the Church there. After their death, Mark, Peter's disciple and interpreter, also wrote down what Peter preached. For his part, Luke, Paul's companion, recorded in a book the Gospel preached by Paul. Then John, the disciple of the Lord, the same who had rested on his breast, also published the Gospel while he was in Ephesus" (Irenaeus of Lyons, Against Heresies, Book III, 1, 1).

Clement of Alexandria (c. 180-200)

Around 180-200, Clement of Alexandria reported: "Mark, a disciple of Peter, while Peter was publicly preaching the Gospel in Rome in the presence of some of Caesar's knights and giving many testimonies about Christ, at their request to leave him a record of the things that had been said, wrote the Gospel that is called Mark's Gospel, from the things said by Peter, just as Luke is recognised as the pen that wrote the Acts of the Apostles and as the translator of Paul's Letter to the Hebrews" (Adumbrationes in epistolas canonicas).

Tertullian of Carthage (c. 207)

Finally, around the year 207, Tertullian mentions that the Gospels were written by the apostles (John and Matthew) and the apostolic men (Luke and Mark): "First of all, I affirm that the Gospel documents were written by the apostles and that the task of promulgating the Gospel was imposed on them by the Lord himself [...]. In short, among the apostles, John and Matthew implanted the faith in us, while among the apostolic men, Luke and Mark reaffirmed it "(Against Marcion, Book IV, 2, 1-2).

These sources provide us with early external confirmations of the four Gospels and their authors. It is also important to note that these references come from very different places: Turkey, Palestine, Italy, France, Tunisia, and Egypt. When the testimonies of ancient witnesses from geographically distant regions all converge, the evidence cannot be stronger.

No tradition or testimony has ever contradicted this attribution, as would have happened if the Gospels had been written anonymously. Even the enemies of Christianity, such as Celsus, recognise that the evangelists are indeed Matthew, Mark, Luke and John. Before the end of the fourth century (and Faustus of Mileve), no one, not even heretics, dared to dispute the attribution of the Gospels.

Manuscript confirmation

It is important to note that all the ancient manuscripts of the Gospels are signed by Matthew, Mark, Luke and John. Brant Pitre, in his book The Case for Jesus, lists them in "The Manuscript Evidence: No Anonymous Gospels", which can be consulted on this page. There are no anonymous copies of the Gospels among the existing manuscripts.

On the contrary, if we look at the Epistle to the Hebrews, we see that the identity of its author was debated from the earliest centuries, because it was a truly anonymous text. Some Fathers of the Church attributed it to Paul, others to Barnabas, others to Clement of Rome, and still others to Timothy, so that Origen of Alexandria, at the end of the second century, gave up trying to find its real author: "As to the author of the Epistle to the Hebrews, only God knows" (Eusebius of Caesarea, Ecclesiastical History, Book VI, 25, 14).

A collective lie?

Finally, if the early Christians had wanted to lie about the authors of the Gospels, why not attribute them to more popular figures? Why not choose two of the twelve apostles who had direct contact with Jesus, rather than Mark and Luke, unknown associates ?

Mark is rarely mentioned in the New Testament as a whole, and not always in a flattering way: he abandoned his first mission in Pamphylia, causing Paul to refuse to take him on another mission (Acts 15:37-40). Since this incident was known, Mark would obviously not have been the best pick for a random attribution of Gospel authorship.

As for Luke, Paul's companion, his first name is mentioned only three times in the whole New Testament(Col 4:14; 2 Tim 4:11 ; Phlm 24), making him a much less interesting choice than other better-known disciples of Paul, such as Timothy (mentioned twenty-five times), Titus (mentioned thirteen times) and Silas (mentioned twelve times). A forger would therefore never have had the idea of attributing the writing of the Gospel or the Acts of the Apostles to Luke unless Luke really was the author.

We can also be sure that forgers would have picked another name than Matthew, who was somewhat an obscure apostle. What's more, his Gospel was written specifically to preach the Good News to the Jews, an affront to them since he was a former tax collector (Mt 9:9), a type of man hated by the Jews, who considered tax collectors to be on the side of the Romans and sinners (Mt 9:11).

In short, apart from John, the names of the evangelists are not those that would have come spontaneously to the mind of a smart forger who would have wanted to establish the authority of these texts. Peter, Thomas or James would have been more attractive and trustworthy authors. It is therefore reasonable to assume that the traditional attribution is correct.

In conclusion

The Gospels are indeed eyewitness accounts, and include two accounts whose authors were in direct contact with eyewitnesses. This considerably strengthens the historical credibility of these texts and demonstrates that they are not myths or late legends, but detailed biographies of the life and teachings of Jesus, based on real facts and filled with first-hand information.

Going further:

The Historical Reliability of the Gospels by Craig Blomberg, ‎ IVP Academic; Second Edition, Revised, Second (October 18, 2007)

More information:

• Jesus and the Eyewitnesses: The Gospels as Eyewitness Testimony by Richard Bauckham, Eerdmans; Second edition (April 28, 2017)
• Video from Testify YouTube channel: Who Wrote the Gospels? 6 Ancient Sources Tell Us.
• Video from Testify YouTube channel: How We Know The Gospels Weren't Anonymous: Early Use.
• Video from Credible Faith YouTube channel : Preparatory Considerations (Part 1 of 3) of William Paley's - A View of the Evidences of Christianity.
• Video from Apologetics315  YouTube channel: Who Wrote the Gospels? by Timothy McGrew.
• Video InspiringPhilosophy YouTube channel : The Reliability of the New Testament (Authorship & Dating).