Contradictions in the Gospels? There are none.
That is to say, if one understands why “they” were written. In early Christianity, there was incredible diversity. There were Sethians, Ophites, Cerinthians, Marcionites, Basilideans, Valentinians, and many other smaller sects with subtle differences in theology and Christology. This is run of the mill sociology though; within any culture there is bound to be variation so it should be no different for a Christianity without an established orthodoxy.
It just so happens that this lack of orthodoxy is the reason behind the Gospels. I’ll just quote Irenaeus (c. 180 CE) in his tome “Against Heresies”:
7. Such, then, are the first principles of the Gospel: that there is one God, the Maker of this universe; He who was also announced by the prophets, and who by Moses set forth the dispensation of the law,— [principles] which proclaim the Father of our Lord Jesus Christ, and ignore any other God or Father except Him. So firm is the ground upon which these Gospels rest, that the very heretics themselves bear witness to them, and, starting from these [documents], each one of them endeavours to establish his own peculiar doctrine. For the Ebionites, who use Matthew’s Gospel only, are confuted out of this very same, making false suppositions with regard to the Lord. But Marcion, mutilating that according to Luke, is proved to be a blasphemer of the only existing God, from those [passages] which he still retains. Those, again, who separate Jesus from Christ, alleging that Christ remained impassible, but that it was Jesus who suffered, preferring the Gospel by Mark, if they read it with a love of truth, may have their errors rectified. Those, moreover, who follow Valentinus, making copious use of that according to John, to illustrate their conjunctions, shall be proved to be totally in error by means of this very Gospel, as I have shown in the first book. Since, then, our opponents do bear testimony to us, and make use of these [documents], our proof derived from them is firm and true.
Here in Against Heresies 3.11.7, Irenaeus argues that the four gospels have authority because the heretics used them first. Though his supposition is that all of these gospels were originally written by orthodox Christians and these heretics simply mutilated their writings. There’s no evidence of this; the four gospels that arrive to us today were written anonymously.
Also implicit here (and in 3.11.8) is that using four gospels as orthodoxy was a novelty when Irenaeus wrote. All of these heretics preceded Irenaeus, thus we have no unambiguous witness to any Christians using four gospels. Ironically, Tatian used a gospel call “Through Four” (i.e. Diatessaron) but was later branded a heretic by Irenaeus (1.28.1). As Irenaeus writes above, these four major heretical Christian sects only used one gospel.
So in the early/mid 2nd century, there were at least four gospels floating around being used independently by heretics. The earliest “orthodox” witness to gospel material is Justin Martyr c. 150 CE, and he seems to be using a gospel harmony as well (rightly so, since Tatian was Justin’s student). According to the Synoptic Problem, Mark was written first which Luke and Matthew used as a source. As I noted in an earlier post, Marcion (c. 140 CE) seems to be the first witness to the actual Synoptic Problem. That there was an original gospel and then a version of this gospel mutilated by the “defenders of Judaism” and Marcion sought to correct this mutilated version (Tertullian, “Against Marcion” 4.4.4). These could only be Mark and Matthew in the Synoptic Problem, with the addition of Marcion’s “anti-Matthew” gospel which was re-Judaized to become Luke.
Also note that Marcion is also the earliest witness to what scholars now know – that the gospels were originally written anonymously:
Marcion, on the other hand, you must know, ascribes no author to his Gospel, as if it could not be allowed him to affix a title to that from which it was no crime (in his eyes) to subvert the very body.
– “Against Marcion” 4.4.2
Of course, Tertullian’s assumption here and all throughout “Against Marcion” was that our current “Luke” was written prior to 70 CE.
Now we have at least three communities: the community using Mark, the community that mutilated Mark because they were defenders of Judaism (Matthew), and Marcion’s community (Luke). Three out of the four major communities that Irenaeus recounts above. And this is why, in actuality, there aren’t any “contradictions” in the gospels – each gospel was meant as a replacement for the other – and they weren’t writing history, but theology. The Matthean community didn’t use both Mark and Matthew, they mutilated Mark to create Matthew. Each gospel writer was trying to write “orthodoxy” and didn’t use other gospels. It’s not until Justin Martyr/Irenaeus timeframe that the Christians who considered themselves the “universal” (catholicos in Greek) church started using all four to cement their universality.
It’s an ingenious plan, to appeal to the lowest common denominator. But once they did this, trying to assert that all four gospels were orthodox, this is where “contradictions” come into play. But they were unaware of why these contradictions were there in the first place due to their ignorance of the textual relationship between these gospels.