We will return to this subject later on, when the consideration of the sources which have contributed to its formation have given us clearer ideas on the matter.
There does not seem to be any valid reason for doubting the accuracy of the tradition which assigns the authorship of this Gospel to Luke, the companion and disciple of St Paul.
The argument of similarity of style, which has been gone into with the utmost care and minuteness, makes it practically certain that this Gospel is by the same writer as the Acts of the Apostles, and that book bears in itself clear indications that it was written by someone who was an actual companion of St Paul during a considerable part of his missionary work.
Nor is the name of Luke sufficiently notable in the history of the first century to make it likely that it would have been selected for this purpose in the absence of a strong tradition in its favour.
It is, therefore, generally admitted by conservative critics of the present day that St Luke was the author, and we need not discuss the matter further.
Date of St Luke's Gospel This admission fixes the date within very small limits, for we are shut down to the probable extent of the lifetime of a companion of St Paul.
Allowing St Paul to have been a little younger than our Blessed Lord, and allowing St Luke to have been twenty years younger than St Paul, we get as the very latest date at which it is at all probable that the Gospel was composed the year A. 90, or thereabouts, while it may, of course, have been written at a considerably earlier time.
On the other hand, there is internal evidence in the document itself which seems to prove that it cannot be earlier than some years after the destruction of Jerusalem, which event took place in the year 70.
Professor Harnack accordingly puts the date of its composition between the years 80 and 90, and there is no reason why we should not accept that date as at any rate that of its final revision, though it still remains quite probable that parts of it were put together at an earlier date.
External evidence means sources outside of the Bible.