Reasons for considering this Gospel in particular
For the purpose of making so complicated a matter clear to you, it will, I think, be best for me, instead of attempting to cover the whole field of inquiry, which is far too large for satisfactory treatment in a single lecture, to confine what I have to say to a single one only of the four Gospels, and to treat that one in more detail than would be possible on any other plan. The best one for this purpose will be the Gospel of St Luke, both because of the variety and interest of the sources which have been used in its composition, and also because of the aid which is given to us in our study of this particular Gospel by the Preface which its author has prefixed to his work, in which he lets us to some extent into the secret of his methods of composition.
With regard to St Matthew's Gospel, it is only a matter of inference and argument that the author made use of sources at all. We could not have been sure that one who had himself been present at the scenes which he describes -- assuming for the moment that the attribution of the Gospel to the Apostle Matthew is right -- would have availed himself of any other source of information than his own memory of the facts. But with St. Luke the case is different. He tells us plainly in his Preface that he was not himself an eye-witness of what he records, but has collected with much care all necessary information from those who were. He depends, that is to say, upon sources which were external to his own personal knowledge of the facts. And these sources were, at least in some cases, written documents. That also he tells us quite plainly, though his meaning has often been misunderstood. "Many," he says, have "taken in hand" to put on record an account of the events which he is about to narrate. And since their accounts were but partial and incomplete, each confined to a single portion of the whole, and did not in any case provide a uniform, continuous, and orderly narrative of the whole story, he proposes to make an endeavour to fill the gap, and for the first time to provide such a complete history, since he has ample and authentic information at his disposal which covers the whole ground, and begins at an earlier period than any which has hitherto been made public.
This interpretation of the Preface of St Luke's Gospel gives a much more intelligible meaning than does the usual explanation that there were already in existence a number of Gospels of a more or less complete character which were known to St Luke, but have since absolutely disappeared, without leaving a trace behind them. It is, moreover, much more in accord with the results of modern research, and also supplies the reason which led St Luke to undertake his task. If it be accepted, we start with an acknowledgment that in this Gospel at least, whatever may be the case with the others, the use of earlier sources of a written character is actually and definitely claimed by the author of the Gospel, as affording evidence for the trustworthiness of his work, and giving an answer to any possible objection that he had not himself been present at the scenes which he describes.