While Luke contributed the most written material in the NT, it could be argued that it remains unclear as to his faith; given the lack of any "personal testimony" in either Luke or Acts. Both books are written seemingly objectively, and while both Luke 1 and Acts 1 begin with a brief introduction (in the first person) to Theophilus, Luke still only specifically states that he "carefully investigated" the things that had been both passed down and claimed to have happened -- regarding Jesus. Nowhere - that I have found - do we see anything that resembles a profession of faith in Luke.
It seems highly unlikely that the Lord would use a non-theist, and given the belief in the plenary inspiration of the authors, we would have to assume the Spirit was with Luke in his efforts...at least at some point along-the-way; no? This has been something nagging at me since my Acts of the Apostles class, but as is customary with many of my courses, not many of my cohorts wanted to venture down the road of 'questioning' a biblical author.
Just a thought -- I would love to hear some feedback and how any of you would answer/refute a claim like this?