Theissen & Merz (via translation by Bowden) advance 3 arguments against the possibility that Luke-Acts was written by a traveling companion of Paul. I will quote their central contentions and respond to them.
- Contradictory accounts of Paul's 2nd trip to Jerusalem
[I]n the Acts account of the life of Paul the second trip to Jerusalem before the Apostolic Council in 11.30; 12.25 contradicts what Paul himself says in Gal. 1.17-2.1. (The historical Jesus: a comprehensive guide p. 32)
- The title "apostle"
Luke denies Paul the title apostle, which was central to his own self-understanding. (ibid)
- Theological emphasis
Genuinely Pauline theology appears only sparsely. (ibid)
- Accounts of Paul's 2nd trip to Jerusalem
This case relies precariously upon an argument from silence. If Paul did not mention a trip to Jerusalem in Galatians, the theory goes, the trip did not happen. This is not at all a defensible position to take.
Paul's purpose in the cited passage in Galatians is not to provide a thorough travel log since his conversion (he skips huge blocks of time), but to present his apostolic credentials (see Gal. 1:11-12; 2:6-9). The alleged contradiction is easily resolved. I will assume here Paul's conversion in AD 35 (others would go +/- a couple years):
- AD 35: Saul's road to Damascus encounter with the Lord, sparking his dramatic conversion to belief in Christ (Acts 9:3-18)
- AD 38: Saul aka Paul travels to Jerusalem and meets with Peter and James the Lord's brother (Gal. 1:18-19). He does not publicly draw much attention to himself in Jerusalem because he knows he is not popular among Christians whom he had persecuted (Gal. 1:22)
- AD 45: Paul & Barnabas deliver donations to the Jerusalem church (Acts 11:30; 12:25). They do not meet with Peter because he is not there (see Acts 12:17--since James the Lord's brother is treated as the man in charge at this time, there is reason to believe other major leaders of the church fled the persecution of Agrippa, not just Peter). Because Paul did not meet with Peter, attend a significant conference, or otherwise establish relevant credentials during this trip to Jerusalem, it is irrelevant to his argument in Galatians and is unmentioned
- AD 49: Paul attends the Jerusalem Conference to discuss the expectations of Gentiles in the church (Gal. 2:1; Acts 15:4)
The accounts recorded by Luke & Paul offer distinct details & perspectives, and were recorded with different agendas in mind, but they are not nearly so irreconcilable as Theissen & Merz claim. Neither are Acts 15:2 & Gal. 15:2 irreconcilable. Paul did what other Biblical examples do in seeking revelation--he reasoned to the best of his ability AND sought inspiration, both of which led him to inquire of the leaders of the church.
- The title "apostle"
The claim is that Paul considers himself an apostle (see the introductory words of his epistles), and that author of Acts does not consider Paul an apostle. This is demonstrably false:
14 Which when the apostles, Barnabas and Paul, heard of, they rent their clothes, and ran in among the people, crying out (Acts 14:14)
Acts does point out that Paul shows deference to the leaders of the church (especially in Acts 15)...but so does Galatians (see Gal. 2:2,9). Paul was not a man to stay quiet when he disagreed with somebody; when Paul's character is considered, there is little difficulty recognizing that the deferent & respectful Paul of Acts 15 & Gal. 2:2,9 is the same man as the outspoken Paul of Gal. 2:11.
Paul respected Peter's authority, but also recognized that Peter was a man and Paul spoke boldly when he believed Peter's actions missed the mark.
Lest we think Galatians shows Paul to be full of himself and unteachable, let us consider that he acknowledges that he was authorized by Peter, James, and John (Gal. 2:9).
Those who believe it is impossible to respect and sustain a leader, while also voicing the occasional strong difference of opinion, must never have participated in the leadership of an organization.
- Theological emphasis
This is the weakest of the arguments, and Thiessen & Merz spend a grand total of 1 sentence on it.
Luke-Acts is clearly written to someone who does not have at his disposal all the core details of Christian history & belief (and Luke says as much in Luke 1:4). Furthermore, there is a notable legal focus in Acts. It records numerous trials, repeatedly cites legal precedent, and records lengthy legal testimony.
Some have argued that the book of Acts was specifically written to aid in a legal defense of Paul (e.g. here). This is a noteworthy difference in purpose versus Paul's letters, which typically address doctrinal controversies among Christians.
As such, it is no surprise that relatively little of Paul's deep theology is presented in Acts. It would be unhelpful to the audience, and it would in fact contradict Paul's own maxim of "milk before meat".
There are only 7 speeches of Paul recorded in Acts, totaling 122 verses (source). 2.5 of those speeches are focused on making a legal defense, and much of the material is repeating the story of Paul's conversion.
The remaining speeches give us less than 70 verses of Paul actually discoursing on theology. Of course that's less Pauline theology than the more than 2,000 verses in his epistles!!!
Thiessen & Merz rely on the popularity of their view: the belief that most of the Bible is late, unreliable fiction is an extraordinarily popular claim among those who wish to believe that the Bible is late, unreliable fiction.
Furthermore, Thiessen & Merz's work is saturated with naturalistic assumptions (see here). These assumptions severely restrict their ability to derive insights from the text.
The Bible is a book that purports to be full of prophecy from beginning to end. In order to objectively evaluate a book about prophecy we cannot start out with the a priori assumption that all prophecy is real, or all prophecy is fake, because doing so restricts the possible solutions we can discover. We would only ever be able to find ideas that align with our worldview; anything outside that bubble would remain forever invisible to us.
To make an a priori assumption on the very topic under evaluation is to arbitrarily select a solution without even evaluating the evidence (not very scientific!). If we start with the premise that this book is a fraud, of course we’ll end up with a conclusion that this book is a fraud—it’s one of our premises! But that’s not an argument, it’s circular reasoning.
When evaluating the texts of Luke & Paul without the restrictive assumptions of naturalism, there is a clear real-world setting for their writings (within their lifetimes!), and the differences in their focus is readily explained by the different purposes they had in writing. There is no reason to conclude that a traveling companion of Paul could not have written Luke-Acts.
Thiessen & Merz also make assumptions about the Synoptic Problem and New Testament chronology that I find highly questionable. For my own work on the subject, rejecting both their favored solution to the Synoptic Problem and their late chronology, see my video series here.