However we conceive of the relationship between Paul and the author of Acts, problems abound. If it is thought that Acts was written by Paul's companion, Luke, we cannot avoid asking why someone who knew Paul well failed to mention that he wrote letters to communities of believers. If we think that Acts was written in the 80s of the first century, we then must deal with an author who wrote before the letter collection was known and was ignorant of a most fundamental fact about his hero. If we move the date of the composition of Acts into the second century, when the Pauline letters must have been available, we face the question of why they were ignored.

In an essay that dealt with questions of this sort, John Knox (1966) noted that, several decades earlier, Edgar Goodspeed (1933) had maintained that the relationship between Acts and the letters of Paul was one of cause and effect and that the publication of Acts had inspired the collecting of the letters. Knox also called attention to an essay written by Morton Enslin (1938), who, contrary to Goodspeed, maintained that the author of Acts both knew and made use of some of Paul's letters. Knox found Enslin's argument for the use of Paul's letters in Acts to be unproven, but he was impressed with what he called “the a priori case for Luke's knowledge of the letters” (Knox 1966: 282).