What is the Synoptic Problem?
The synoptic problem concerns the problem of knowing the kind of literary interrelationships responsible for the similarities existing between the Gospels of Matthew, Mark and Luke. The majority of scholars believe that between these three Gospels (the three 'synoptic' Gospels, or 'look-alike' Gospels) there are many more 'agreements' in terms of their shared content, order and wording than should be expected amongst independently written accounts (compare the fourth Gospel 'According to John' which shares very little overlapping content, order or wording with the 'synoptics') and so it is that these similarities require some kind of literary explanation.
Increasingly since the seventeenth century various scholars have sought to find the most plausible explanation for the kind of similarities between Matthew, Mark and Luke. Such explanations are known as Gospel 'source theories' (or 'hypotheses'). However, admitting the existence of the synoptic problem is controversial in certain conservative quarters because, for theological reasons, some Christians do not believe that any kind of literary relationship could exist amongst the Gospels, as though such would be to charge the Gospel authors with plagiarism, or even potentially undermine the perceived historical reliability of the Gospels as supposed independent compositions (i.e. independent historical witnesses). However no such independence is claimed by the Gospels themselves. In fact the first verse of Luke refers to many others already having put together a written account (Lk 1:1) which probably suggests the author had either read other accounts, or at least heard them read out.
Early History of the Synoptic Problem
Gospel source theories have only been developed since the seventeenth century, probably in parallel with changing conceptions of (1) what an 'author' is, (2) perceptions of what 'history' is and (3) the type of literature the Gospels represent. Consequently 'literary dependence' (one Gospel author utilizing another Gospel as a source), is a relatively modern notion. For most of Church history the four Gospels were perceived as having been written somewhat independently of one another. Church theologians gave little thought to the kinds of written sources available to the Gospel authors or the kinds of writing procedures such 'authors' would have used in the composition of their respective Gospels. When the fourth century theologian Augustine acknowledged that Mark reads like an abbreviated version of Matthew, it is not clear whether he was implying anything about the use of Matthew by the author of Mark, or whether he is simply stating the obvious point that there is much in common between the two and that Mark is the briefer of the two.
Tatian in the late-second century solved the 'problem' of having four different Gospel accounts by combining them into one long account. This 'mixed' Gospel was especially popular in the Syriac version until about the fourth century.
Origen in the third century saw that the differences between the Gospels were due to the different ways the Evangelists modified their narration of historical events for spiritual reasons, at the expense of historical accuracy.
Until the seventeenth century it is difficult to find any discussion of Gospel interrelations beyond the superficial observation of when in time each was written, i.e. the likely order of composition.
Prior to the seventeenth century the problem was thought to be the differences between the four Gospels (the similarities were not considered to be the problem). So Church theologians thought they needed to explain the differences so as to defend the unity of 'the Gospel' and to explain away the differences (or to 'harmonize' apparent discrepencies) between the Gospels, as such differences were thought to present a theological vulnerability to the validity of the Church's teachings as though the 'truth' of the Gospels depended on all four Gospels being in total agreement in all details. The 'problem of difference' had often been dealt with in various ways, with the problem of 'chronology of events' gaining much attention. For example, in the sixteenth century Calvin's general solution to the problem of different chronologies between the Gospels was stated in this way:
No fixed and distinct order of dates was observed by the Evangelists in composing their narratives. The consequence is, that they disregard the order of time, and satisfy themselves with presenting, in a summary manner, the leading transactions in the life of Christ. They attended, no doubt, to the years, so as to make it plain to their readers, in what manner Christ was employed, during the course of three years, from the commencement of his preaching till his death. But miracles, which took place nearly about the same time, are freely intermixed.
Calvin disregarded the extreme approach of Ossiander whose harmony (1537) had Jesus tested by Satan three times, with three healings of blind men near Jericho, three centurions’ sons healed, three anointings of Jesus by three different women, three cleansings of the temple, and had Jesus betrayed by Judas twice.
So Calvin decided that only one Gospel at any given time need have given the correct order of events.
Similarly, Martin Chemnitz (1522-1586) believed that only when more than one gospel agreed in their order of episodes, could we suppose that this indicated the actual chronology of events. Chemnitz believed that the Sermon on the Mount found in Matthew and the Sermon on Plain found in Luke were actually two versions of the same sermon.