When we look at the gospels of Matthew and Luke we can see obvious parallels.  That is because Matthew and Luke drew upon Mark for most of their gospels.  Matthew's gospel contains ninety percent of Mark's work.  Luke's gospel contains 50% of Mark's work.  This seems to hint that Mark was their common source.   

But how about the other ten percent of Matthew's gospel and the other 50% of Luke's gospel that didn't get its information from the Marcan Source (Mark's gospel)?  Matthew and Luke agree verbatim.  How is this possible?  There must be some other source to explain this.  

The likelihood of the existence of Q has been made stronger by the discovery, in 1945, of the Gospel of Thomas, which is also collection of sayings without reference to the "story" of Jesus' life and works. Thomas has forty-seven parallels to Mark, 40 to Q, seventeen to Matthew, four to Luke and five to John. Some conclude that both documents began in communities of Jewish Christians in Judea and Galilee between the time of Jesus' death and the writing of the four main gospels.

The four main gospels seem to think less about Jesus coming again, and more about what they call the "Kingdom of God". There is a well-supported argument for assuming that by "Kingdom" they meant allowing God's sovereign rule be the guidelines in their thinking about life and the context of the time period when they lived. As we know from Paul's later letters, by around the year 65, Christians started wondering if Jesus would come back as some had predicted. This makes a date for Q of later than 65 yrs. less likely. Some think that the story of the fight with the devil in Q is talking about a situation in 39 when there was a huge demonstration against the erection of a statue of the Emperor Caligula in Jerusalem [1]. If that's what it is, the Q material likely came into being after that time.

Mark's Gospel contains no Q material but is used as a source by both Luke and Matthew. Amazingly, both Luke and Matthew also use Q versions of the sources used by Mark in his gospel.

One example is:

Mark 4.25 is used in Matthew 13.12 and Luke 8.18. A similar version from Q also occurs in Matthew 25.29 and Luke 19.26.

This is another firm confirmation that Q was just one source and that it was used by both Matthew and Luke separately of Mark. The determination that Q was as a now-lost written source of some sayings of Jesus has stood firm after more than a a hundred years of scrutiny with amazing resiliance.

If one agrees that Q is a now-lost written record of "what Jesus really said", then 'Q' should be regarded as a long-missing record of the words that Jesus actually said.

·Many have dismissed thelikelihood that we can know what Jesus really said. The large time between his life and the first of the gospels (probably Mark) is, they say, too long for oral material to still be right. The changes that naturally happen through time and distance of what Jesus really said would be considerable - Some propose that the gospels contain the verbatim words of Jesus. The nature of Q renders this not likely. First, it has obvious signs of having been revised into a written version from the casual way people generally speak. More often than not, (most-not all) people only speak with correct grammar if they are reading a prepared script. Second, the gospel authors have 'borrowed' the Q material into their own theological schemes. There's no way of determining whether the writer or editor of Q did not do just that. It's reasonable to assume he did.

Several other assumptions can be validly drawn from the evidence [4]:

·Not only was Q originally a written source, but it was written in Greek. Attempts to find Aramaic in the text have been unsuccessful.

·Q was not written as a single piece. Rather, it was a collection which was added to from time to time. Some changes could have been made by later, non-Galilean sources - though the proof for this is fairly weak.

·Those who are not familiar with the scholarly work of the past twenty years may react to Q with some unhappiness. This is understandable.  We have, represented by the gospels, not a channeled hand-me-down from God via an undefinable process of revelation, but a truly human interpretation of events (rather than a record) put together in a normal way for those times. 

Our knowledge today about Jesus is therefore subject to all the strengths and limitations of normal human processes by which information is shared from person to person.
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[1] The Historical Jesus,  G Theissen && A Merz, SCM Press, 1998
[2] Midrash and Lection in Matthew, SPCK, 1974.
[3] The New Testament, N Perrin && D C Duling, Harcourt, 1974
[4] After Archaeology and the Galilean Jesus, J L Reed, Trinity Press, 2002