A great number of theories have been developed regarding the sources used by the author (Conzlemann, xxxvi-xl). Many of these theories are colored by presuppositions regarding the historicity, authorship, and purpose of Acts (Guthrie, 363-77). Even for those who presume that the author was a companion of Paul, the question of sources is important. Obviously the author would have firsthand knowledge of the “we sections,” and secondhand knowledge through Paul for those narratives where Paul was present (6:7-8:3; 9:1-30; 11:19-30; 12:1-24[?]; 12:25-28:31 as well as much of 2:1-6:6, if Paul was dwelling in Jerusalem for some time prior to his emergence as leader of the persecution). But the question remains: Where did Luke get his information for those portions where Paul was not present?
Luke tells us in his gospel that “many have undertaken to compile a narrative of the things which have been accomplished among us, just as they were delivered to us by those who from the beginning were eyewitnesses” (1:1-2rsv). This description suggests the existence of both written and oral accounts. Elsewhere the writer implies that he was present with Paul in Palestine for two years during Paul's imprisonment in Jerusalem and Caesarea (Acts 21:17 [“we”]; 27:1 [“we”]). This provided time to meet and talk with those who were present in the early days of the Christian movement prior to Paul's involvement.
Thus, presumably, the writer had available both written and oral accounts of the events, speeches, sermons, and trials in the first part of Acts. The style of the speeches in the first part is inferior to the narrative material in the rest of the book. Moreover, these briefer speeches are perfectly consistent with their setting, suggesting that the author was faithful to his sources (Bruce, 18-21).