Liberals typically deny that any of the four gospels preserve eyewitness testimony. They deny the apostolic authorship of Matthew and John. They date all four gospels as late as they can to put them as far as possible out of reach of living memory. And they also postulate a lengthy phase of fairly creative oral transmission before redactors even committed this tradition to writing.
They basically view the gospels as allegories for the circumstances of the church at the time the gospels were “really” written. According to them, redactors concoct speeches and incidents to furnish a backstory for church doctrine and practice.
Conversely, conservatives traditionally regard two of the four gospels as having been written by apostles. As such, they transcribe direct eyewitness testimony. Conservatives also think Mark contains eyewitness testimony, because Mark is channeling the witness of Peter. And they think Luke contains eyewitness testimony drawn from his oral and written sources.
Without denying that Mark is passing along eyewitness testimony which he heard from his circle of informants, I’ve also argued that since he was a native of Jerusalem (Acts 12:12), Mark was probably an eyewitness in his own right whenever Jesus came to town.
It’s usually assumed that although Luke was an eyewitness to some events recorded in Acts, he was not an eyewitness to any events record in his gospel. However, the wording of his prologue doesn’t actually say that, and may even point in the opposite direction.
One ambiguity is the way he includes himself in the statement about “the things accomplished among us” (Lk 1:1). Since that statement introduces the gospel, you’d expect that statement to cover at least some events recorded in the Gospel account.
Some commentators think it anticipates the “we-sections” in Acts. But while the statement may well take a long-range view, it would be odd for Luke to introduce his Gospel with this self-inclusive reference, only to drop it for the entire gospel narrative, and then expect his audience to pick up on claim when they finally got around to Acts.
And that’s not the only potentially self-referential statement of its kind in the prologue. As one commentator notes,
“Eyewitnesses” (autoptes). The word, which is absent from the LXX, is comparatively rare in Greek writers, and tends to occur in a limited number of specialist contexts. The principal meaning of autopsia is…"seeing something for oneself.” It was thus used by geographers of the knowledge of foreign lands acquired by personally visiting them, or from those who had done so…In its rare occurrence in the papyri an autoptes is someone commissioned to investigate or inspect–an observer or overseer. In scientific, especially medical works it belongs closely with the author’s claim to experience, and with the necessity of basing the science on the observation of empirical data rather than dogma…The idea was given a special slant by Thucydides (though not the word, which he never uses), when in his account of his sources and method in his preface (I, 22), he refers to his presence at some of the events he records, and to his ability to examine witnesses, which meant that his history had to be for the most part of contemporary events. This established a convention, and is repeated with or without the word autoptes, sometimes in prefaces and sometimes with the narrative itself, of a succession of historians…In which tradition Luke stands here is difficult to say, since autoptai has no object. If this is to be supplied for “the things which have been accomplished among us” from the previous verse, then the claim could be for eyewitnesses as the basis of the accounts both of the Gospel and of Acts, C. F. Evans, Saint Luke (Trinity Press 1990), 126-27.
“For some time past” (anothen), characterizing Luke’s activity of following, not that which he had followed. The word occurs again in Luke-Acts only at Acts 26:5, also in proximity to ap’arches="from the beginning.” The two could be synonymous…In that case Luke would be stressing that his personal activity and familiarity with the events went as far back, and was as original, as that of the eyewitnesses and ministers of the word. But they could be distinguished, as, in the view of some, in Acts 26:4f…Luke’s claim would then be to accurate personal knowledge of the Christian movement from a long time back, ibid. 131.
Richard Bauckham (Jesus and the Eyewitnesses, 116-24) and Craig Keener (The Historical Jesus of the Gospels, 91f.) also have some useful analysis of the prologue's terminology.
So Luke may well be telling the reader that he was a sometime witness to events in the Gospel as well as his history of the church. In that case, his Gospel has a foundation in his firsthand observations, supplemented by written sources (e.g. Mark) as well as interviews he conducted with other eyewitnesses.
Of course, we don’t know the circumstances under which he might have had occasion to observe certain incidents in the Gospel, although it’s easy to speculate. Perhaps he made his living in Jerusalem as a physician who treated military detachments stationed there or thereabouts. That might also explain his friendship with Theophilus, assuming that Theophilus was a centurion or some other Roman official (e.g. procurator) connected with the occupation of Palestine.
Although he wasn’t commissioned in the sense that Jesus commissioned the apostles, and though he didn’t have the daily contact with Jesus which the Twelve had, that doesn’t mean he was in no position to see what he reports.
There were many eyewitnesses to what Jesus said and did besides the Twelve. Observers who were present at some event or another–depending on where they happened to live, or how much leisure time they had to follow Jesus around.
My point is not to prove anything–much less demarcate his sources. My point, rather, is that it’s inaccurate to claim Luke could not have been on the scene at some the events he relays in the Gospel bearing his name. The prologue doesn’t rule that out. If anything, he includes himself in the narrative–as a sometime spectator or participant.