But most scholars tend to agree that Q probably existed and, for the sake of this post, I'll assume this position. You have to keep in mind that all our information about this hypothetical document comes from the Gospels of Matthew and Luke, that is, from these two Gospels themselves (not from Gospels-independent sources).
Since the hypothetical Q document is earlier than the Gospels (hence, it passes the criterion of date), it has been used (or abused) by some liberal scholars in order to create a portrait of Jesus which fits with their anti-Christian ideas about him.
In this post, using the example of Burton L. Mack, I'll show how the Q document is unjustifiably misused by anti-Christian scholars in order to deny the traditional view of Jesus. You'll see that a large part of the belief that "Q" provides an antidote against the traditional view of Jesus is based on bad scholarship, illogical thinking, hidden anti-Christian assumptions, wishful thinking and a negative use of the criteria of authenticity (specially of the criterion of date which is misused as a necessary criterion, not as a sufficient one).
In his book The Lost Gospel: The Book of Q and the Christian origins, Q scholar Burton Mack wrote this:
"The first followeres of Jesus did not known about or imagine any of the dramatic events upon which the narrative gospels hinge. These includes the baptism of Jesus; his conflict with Jewish authorities and their plot to kill him; Jesus' instruction to the disciples; Jesus' transfiguration, march to Jerusalem, last supper, trial, and crucifixion as the King of Jews; and finally, his resurrection from the dead and the stories of the empty tomb. All of these events must and can be accounted for mythmaking in the Jesus movements, with a little of help from the martyrology of Christ, in the period adter the Roman-Jewish war. Thus the story of Q demostrates that the narrative gospels have no claim as historical accounts" (p.247)
The above argumentation is a fine example of bad scholarphip and technical incompatence on behalf of an anti-Christian agenda. As a preliminary point, note Mack "negative" conclusions regarding the traditions not found in Q. It is implicitly assumed that whatever is not found in Q (specially the distintives Christian views) are not historical and hence were a later invention.
Obviously, objective truth-seekers will demand that this anti-Christian assumption needs some kind of positive evidence, which is not found in Mack's book.
Let's examine Mack's scholarly mistakes in more detail:
1-One of the criteria of authenticity used by scholars is the criterion of date: All things being equal, we should privilege earlier sources over later sources (because an earlier source is likely to report more accurately the historical facts than later sources).
Note that the correct use of this criterion is the positive one: If a saying is attested in a earlier source, then it increases the probability of that saying being authentic. But if the saying doesn't pass this criterion, you cannot conclude that the saying wasn't authentic (because it could pass OTHER criteria like multiple attestation, embarassment, etc.) So, this criterion provide a sufficient, not a necessary, condition for authenticity and historical credibility.
But note that Mack is using the criterion negatively, that is, as a necessary condition for credibility and authenticity of the facts relative to Jesus. Using the criterion in this wrong way, he can conclude happily and negatively: "Q demostrates that the narrative gospels have no claim as historical documents" and he can freely deny facts multiply attested like the empty tomb.
In other words, using the criterion of date as a necessary (negative) condition, Mack is free to disregard as non-historical later documents which report sayings, which pass positively other criteria of authenticity like multiple attestation, embarassment, dissimilairy, etc..
Can you see Mack's trick here? You have to have a correct understading of the use of the criteria of authenticity in order to see how Mack is taking in his readers. This will become evident in the next points:
2-If the criterion of date regarding the Q document is used negatively, as a necessary condition for authenticity and historicity, then the most indisputable fact about the historical Jesus, namely, his crucifixion, will have to be denied and considered to be non-historical, because Q doesn't clearly shows Jesus' crucifixion.
But Jesus' crucifixion is multiply attested, even in sources outside of the New Testament. According to atheist John Dominic Crossan: "That he [Jesus] was crucified is as sure as anything historical can ever be" (Jesus: A Revolutionary Biography, p. 145).
And keep in mind that Crossan accepts the crucifixion, not on the grounds of the NT evidence, but on the grounds of the LATER non-Christian sources of Josephus and Tacitus (see Crossan's book, The Historical Jesus, p. 372).
In other words, since as an athest Crossan is skeptical of the NT material, he accepts the crucifixion on the grounds of non-Christian sources, which make the crucifixion multiply attested (even if they don't pass the criterion of date, because the Gospels' reports of the crucifixon are EARLIER than those non-Christian sources).
Can you see how liberal scholars use the criteria of authenticity both negatively (e.g. Mack regarding the criterion of date) and positively (e.g. Crossan regarding the criterion of multiple attestation... even though Crossan also uses this criterion negatively, for example, in order to deny the passion and resurrection narratives in the Gospels).
This is a subtle and very technical but (once properly understood) almost irrefutable example of bad scholarpship put at the service of an anti-Christian agenda.
3-The traditions about the empty tomb pass, positively, the criterion of embarassment and multiply attestation (e.g. in Paul) and hence it is likely to be historical. However according to Mack, since the criterion of date regarding Q is necessary for historicity and it doesn't contain reference to the empty tomb, then the traditions of the empty tomb were later inventions, a mythmaking of Christians. (Also for Crossan, the empty tomb was an invention by Mark; here Crossan clearly disregards the positive use of the criterion of embarassment and mutiple attestation regarding the empty tomb but in other cases Crossan himself uses positively, that is as a sufficient condition, the same criterion, for example regarding the crucifixion).
Can you see how the misuse of the criteria of authenticity allows these scholars to select only the facts that fit with their preconceived anti-Christian view of Jesus? They use these criteria selectively and inconsistently in order to create a view of Jesus which fit with their anti-Christian ideologies.
4-Likewise, since the Q document doesn't contain Jesus' resurrecton, the misuse of the criterion of date by Mack allows him to conclude that the resurrection wasn't known or imagined by the "first followers of Jesus". However, the resurrection appearences pass positively the criterion of multiple attestation (being reported not just in the Gospels but in Paul too).
Again, it is Mack's negative use of the criterion of date regarding Q which allows him to consider non-historical the distintives Christian elements of the historical Jesus which pass, positively, OTHER criteria of authenticity (like multiple attestation, embarrasment, etc.).
5-In Q, there is evidence that Jesus saw himself as the unique son of God. The saying in Matthew 11.27: "All things have been committed to me by my Father. No one knows the Son except the Father, and no one knows the Father except the Son and those to whom the Son chooses to reveal Him" is a Q saying (see Luke 10:22).
However, the atheist and religious pluralists scholars in the Jesus Seminar (and some other liberal scholars) don't accept that Jesus regarded himself as the Son of God even if this view is made explicit in the Q document and therefore passes positively the criterion of date (plus other criteria like the criterion of dissimilarity, as I've argued here, which reinforces the authenticity of this saying).
In order to deny this Q's view of Jesus, these scholars are forced to speculate about "layers" in Q, so they can claim that some layers are "earlier" (and hence more reliable) than "later" layers (and hence, less reliable) of Q. This approach is largely speculative and provides too much room for the scholar's own personal assumptions to go uncontrolled (for example, seeing a "sage" layer as earlier precisely because it portraits a purely "sage" view of Jesus, which is clearly question-begging).
Moreover, constructing speculation and hypotheses on other purely speculative hypotheses is clearly a procedure unlikely to produce historically accurate results. But the scholars who prefer this kind of procedure are pushed in this direction in order to deny the traditional portrait of Jesus which they want to disbelieve.
6-As an example, Mack and other anti-Christian scholars reconstruct an "earlier" layer of Q in which Jesus appears to be noneschatologcal (Non-eschatology is also essential in the work of the Jesus Seminar). So, Q sayings which are clearly eschatological (e.g. Jesus rejecting the wicked at the judgment at Matt 7:21-23 and Luke 13:26-27, or the powerful endtime judge burning the wicked at Matt 3:7-12 and Luke 3: 7-9/ 16-17) are considered to be non-historical sayings.
This point shows clearly the wishful thinking of these liberal scholars: For theological and ideological reasons, they don't want a Jesus like that. Therefore, they mutilate Q itself in order to leave only the sayings of Jesus which fit with their anti-Christian view of Jesus as a non-eschatological, non-divine, purely wisdom teacher/Cynic phlosopher type of man which is palatable to a secular age in which religious pluralism, New Age-mystical spirituality and atheism is rampant, palatable and politically correct.
Since they want to believe in a New Age type of Jesus (a Jesus which is not unique or special in any divine sense, but a Jesus on a par with other important spiritual teachers. See the work of Marcus Borg for this kind of view). they try hard to misrepresent the evidence (including the evidence in Q) in order to give plausibility to their position.
Can you know see why these people, consistently and predictably, cannot accept Jesus' resurrection and, like Borg, force false interpretations on the evidence for it (e.g. in 1 Cor 15)? Can you see why these false and eccentric views by these scholars cannot be euphemistically considered as mere "tendencies" or "inclinations" or "excess" or " simple mistakes", but the logical and necessary consequence of their anti-Christian pressupositions which pervade their work? A resurrected Jesus doesn't fit well in the pluralist view of Jesus that they have made and this tension, clearly perceived by them, is eliminated through a selective use of the evidence supported by a clever and astute misuse of the criteria of authenticity.
So, religious pluralist Marcus Borg can happily claim "Imaging Jesus as a particular instance of a type of religious personality known cross-culturally undermines a widespread Christian belief that Jesus is unique, which commonly is linked to the notion that Christianity is exclusively true and that Jesus is "the only way." The image I have sketched views Jesus differently: rather than being the exclusive revelation of God, he is one of many mediators of the sacred. Yet even as this view subtracts from the uniqueness of Jesus and the Christian tradition, it also in my judgment adds to the credibility of both." (Meeting Jesus Again for the First Time: The Historical Jesus and the Heart of Contemporary Faith, p. 37)
A Jesus who is "one among many" is precisely that kind of Jesus which is palatable to a contemporary culture (like our) in which religious pluralism, New Age spiritualities, etc. are ruling. (Q sayings and other authentic sayings of Jesus in which he implies that he is "the only Son of God", God's final eschatological agent, Jesus' predicting his own resurrection and so forth, obviously cannot have place in this religious pluralistic, New Age kind of portrait of Jesus). It is not surprising that the work of liberal scholars, specially those in the Jesus Seminar, have been so influential in popular culture.
Understanding all of this is absolutely essential to expose the portrait of the historical Jesus made by the Jesus Seminar (they are the champions of the misuses of the criteria of authenticity) and people like Burton Mack, Crossan, Borg and many others.
In conclusion, the Q document is an hypothetical (not proven to exist) source. But even assuming its existence, it is only the misuses of the crterion of date (as a necessary condition) plus the misuses of OTHER criteria of authenticity (whch are used positively or negatively, depending on which facts the "scholar" in question wishes to select in order to create a Jesus in his own image) plus speculative ahd largely arbitrary division of Q into "layers", which underlies the beliefs that the Q document provides a different view than the traditional view of Jesus. The latter is a non-sequitur which is testimony of bad scholarphip and, above all, of ANTI-CHRISTIAN PREJUDICES AND WISHFUL THINKING.
Q is NOT the only source about the historical Jesus, hence it is not sufficient to the reconstruction of him. As consequence, it is unwarranted and strongly biased to think that the information contained there suffices to conclude that the Q document is at variance with the traditional view of Jesus, because the information contained in other sources can qualify, interpret, put into context or develop the information contained in Q.
Amazingly, liberal scholars seem to be unware of or incapable to perform this contextual examination of the evidence, as evidenced for example by Marcus Borg in his thoughtless, unjustified, illogical, incompetent and purely prejudiced obsession to read 1 Cor 15 as an "explicit denial" of bodily resurrection by Paul. See discussion here, here and here).
For a brief explanation of the misuses of the criteria of authenticity by Bart Ehrman, wacth very carefully (and think hard about it and compare with the work of Ehrman and the Jesus Seminar) the following lecture on Ehrman by William Lane Craig: