The Gospels were written too early after the facts to be legends

The canonical Gospels were written very soon after Christ's death (unlike the apocryphal Gospels, the earliest of which were written between the late 2nd and early 3rd centuries). According to a number of experts, the final composition of the Synoptic Gospels (written by Matthew, Mark and Luke) most likely dates from before 70 A.D., which is less than forty years after Jesus' death (around the year 30 AD).

According to some other New Testament exegetes, the Gospels were written between 70 and 90 AD. Even if this were the case, the dating would pose no major problems in terms of historical accuracy, since the eyewitnesses (Matthew and John), as well as their close collaborators (Luke and Mark), were probably still alive at the time. The authors of the Gospels were therefore capable of recording the facts of Jesus' life and preaching. But more recent exegetical studies suggest that the Gospels were written even before that window.

Voûte centrale du Panthéon royal (vers 1149) © GNU Free Documentation License / José Luis Filpo Cabana

Reasons to believe:

New Testament scholars are particularly insistent on the fact that none of the four Gospels mentions the destruction of the Temple in Jerusalem that occurred in 70 AD. This would mean that the Temple had not yet been destroyed when the evangelists wrote their accounts.
It is possible to date Luke's Gospel and affirm that it was written before the first Epistle to Timothy (written around 65 AD), because 1 Timothy (5:18) quotes Luke (10:7) and refers to it as "Scripture".
The language used in the Gospels and Acts of the Apostles indicates that they were written prior to the first persecutions of Christians. We know from the Roman historian Tacitus that the first great persecution of Christians began under Nero in the year 64 AD.
Finally, there are many arguments in favor of dating the book of Acts to the early 60s (precisely to 62). Since Luke’s Gospel, the third one to be written, after Mark and Matthew, was written before Acts, it can be dated no later than 62, and therefore the Gospels of Matthew and Mark date from even earlier than that date.

Summary:

Firstly, it is important to note that none of the Gospels mention the destruction of the Temple in Jerusalem in the year 70. This is significant, as the evangelists would have expressed their joy at seeing this prophecy fulfilled. If the Gospels had been written after A.D. 70, we can well imagine the evangelists commenting on it: "Jesus announced it and this prophecy was fulfilled X years later", in order to support their hero's messiahship. But they do no such thing; they simply transcribe Jesus' words announcing that destruction is coming. Historian Charles Cutler Torrey remarks: "It might have been conceivable that one evangelist writing after the year 70 could have omitted to allude to the destruction of the Temple by the Roman armies [...], but that three (or four) evangelists could have forgotten to allude to it seems quite incredible" (C. C. Torrey, The Apocalypse of John, New Haven, Yale University Press, 1958, p. 86).

Secondly, we know that Luke's Gospel was written before Paul's first Epistle to Timothy, because verse 1 Tim 5:18 quotes Luke 10:7 and refers to it as "Scripture". This means that Luke's Gospel existed before Paul wrote this epistle, and was already considered "Scripture". Paul was beheaded around the year 65 (some scholars say 67), so he must have written his epistle before that date, which would put Luke's Gospel at the beginning of the 60s.

Thirdly, the language used in the Gospels and the Acts of the Apostles indicates that they were written at a time before the persecution. The first major persecution of Christians took place under Nero in 64: they were crucified, burned and thrown to starving dogs, as reported by the Roman historian Tacitus (Annals, 15: 44). It is completely unthinkable that Luke would have omitted to mention these atrocious persecutions in the Book of Acts, if they had already taken place. This would have been an ideal opportunity to emphasize the bravery of the Christians martyred for their faith in Jesus. What's more, when we look at Christian writings dating from after 64, we often notice that they show a certain anger towards the Romans who persecuted them. None of this is to be found in the Gospels or Acts of the Apostles, which, on the contrary, have a friendly attitude towards Rome (Acts 21:28; Acts 27:3; Mk 15:39; Mt 8:5-13; Lk 7:1-10; Acts 26:24-27). Now, to speak well of the Romans after the great persecution would be equivalent to speaking well of the Germans just after 1945. It is therefore likely that the Gospels and Acts were written before the great persecution of 64.

Finally, the last reason invoked by historians provides further precision. It begins by dating the Acts of the Apostles, and then deduces the date of the Gospels. The end of Acts is centered on the figure of Paul recounting his various journeys, before coming to an abrupt end when Paul arrives in Rome. The verse in Acts 28:30 states that, once in Rome, Paul stayed there for two years before his trial. And the book of Acts stops almost immediately, without giving us the verdict of the trial, or even mentioning Paul's death, which took place around 65. The logical explanation for this is that the trial had not yet taken place. Indeed, given the suspense culminating at the end of Acts, it would have been logical for Luke to mention whether Paul had been condemned or not. If he had been condemned, Luke would surely have taken the opportunity to describe his glorious martyrdom and suffering for Christ; and if he had been released, Luke would surely have mentioned his glorious struggle against the tribunal. The most plausible interpretation of this abrupt halt in the writing of the Acts of the Apostles is that the trial was in progress and Luke could not yet give the outcome.

Liberal exegete Adolf von Harnack comments: "Throughout eight whole chapters St Luke keeps his readers intensely interested in the progress of the trial of St Paul, simply that he may in the end completely disappoint them — they learn nothing of the final result of the trial! [...] The more clearly we see that the trial of St Paul, and above all his appeal to Caesar, is the chief subject of the last quarter of the Acts, the more hopeless does it appear that we can explain why the narrative breaks off as it does, otherwise than by assuming that the trial had actually not yet reached its close. [...] If St Luke, in the year 80, 90, or 100, wrote thus he was not simply a blundering but an absolutely incomprehensible historian! [...] We are accordingly left with the result : that the concluding verses of the Acts of the Apostles ', taken in conjunction with the absence of any reference in the booh to the result of the trial of St Paul and to his martyrdom, make it in the highest degree probable that the work was written at a time when St Paul’s trial in Rome had not yet come to an end." (Adolf von Harnack, The Date of the Acts and the Synoptic Gospels, 1911).

This would put the Acts of the Apostles probably two years after Paul's imprisonment (around 60), which would put it at 62.

It should also be noted that Acts makes no mention of the martyrdom of James the Just, which also took place in 61-62, nor that of the other hero of Acts, Peter (around 64). If Acts had been written later, it's hard to see why its author (Luke) would have wanted to leave the reader hanging, without mentioning Paul's fate and omitting the martyrdoms of Peter and James the Greater altogether, especially since the martyrdoms of Stephen (Acts 7) and James the Less (Acts 12:1-2) are mentioned. Nor does Acts mention the war between Jews and Romans in 66, or the persecution under Nero (in 64), or the destruction of the Temple in Jerusalem. All of these elements should be enough to defeat arguments in favor a later date for Acts (around the 80s)!

The dating of the Acts of the Apostles helps to clarify the dating of the Gospels. Indeed, we know that Acts was written after Luke's Gospel, as both are addressed to the same person (Acts 1:1; Luke 1:1-4) and Luke tells Theophilus that it is his second book. So Luke's Gospel was probably written before 62, too.

Finally, let's look at the other Gospels. In academic circles, the study of the order in which they were written is known as the "synoptic problem". For more than half a century, experts have believed that Mark's Gospel was written first, as Matthew and Luke seem to borrow elements from it. Exegetes also believe there's a good chance Luke’s Gospel was written after Matthew and Mark, since Luke begins his Gospel by pointing out that other Gospels were written before him (Lk 1:1-3). And since Luke was written in the very early '60s (as we've just seen), it follows that Matthew and Mark were probably written in the '50s. John A. T. Robinson, far from being a conservative, even suggests dating Mark to around 45.

Thus, we have good reason to believe that the Synoptic Gospels were written between the mid-50s for Mark (although some parts are older) and the early 60s for the Greek versions of Matthew, Luke and Acts. In all cases, we're talking about dates well before the year 70.

The thesis of legendary accounts is thus reduced to nothing. For a legend to appear, the eyewitnesses (and their children and grandchildren) must have been dead for a long time, leaving enough time for the account of an event to be greatly modified and to become part of the collective imagination. The Gospels and Epistles were written far too early for this to happen. Experts estimate that the reliability of Oral Tradition can last nearly two hundred years (Cf. Marlene Ciklamini, "Old Norse Epic and Historical Tradition", in Folklore and Traditional History, 2018). The Gospels therefore have more than enough margin to preserve the authentic words of Jesus, which considerably strengthens their historical credibility.

Matthieu Lavagna, author of the book Soyez rationnel, devenez catholique !