The discrimination between QMt and QLk and the original Q makes unnecessary a good deal of the work that has heretofore been done toward determining the primary and secondary traits in Mark and Q respectively. Assuming that Mark used either the original Q, or as near to the original of that document as we can yet get, the recensions used by Matthew and Luke would be perhaps thirty, certainly twenty, years later than that used by Mark. In the fifty or more verses of Mark that appear to have stood also in Q, there is nothing that can be shown to be later than the year 70 (the date generally assigned to Mark). There is nothing to suggest that the author had witnessed the destruction of Jerusalem, or the events immediately leading up to it. The presence of the same material in Mark and Q is demonstrated by the agreements of Matthew and Luke against Mark, or by the deviations of the one or the other of them from the Marcan form of a saying, in such way as not to admit of explanation except by the assumption of two sources (Mark and Q) in the hands of Matthew and Luke. In other words, if Mark did use Q, but if he used the same text of it as was used by Matthew and Luke, and if the three followed Q with equal faithfulness, in all such instances Q would fail to appear, since both Matthew and Luke would appear to be following only Mark. It is therefore where there are deviations of either Matthew or Luke from Mark in sections where the other follows Mark closely, or where there are agreements of Matthew and Luke against Mark in sayings-material, that the presence of Q behind Mark can be detected.
Upon the hypothesis of Q without QMt or QLk, the argument by which the use of Q by Mark, as against the use of Mark by Q, was proven, consisted of picking out the primary and secondary traits in Mark and Q respectively, and of showing that the primary traits were in Q and the secondary in Mark. But this was very difficult to do, so long as, e.g., the Peter incidents peculiar to Matthew, or the conversation during the baptism, were attributed to Q. For these were indisputably secondary. If the priority of Mark was to be maintained, all such traits had to be removed from Q and assigned to the evangelist or to some special source.