Modern scholarship accepts that the Gospel of Luke and the Acts of the Apostles were written by the same author. But a more subtle question is whether that author started out to write one work in two volumes or if the idea of writing Acts occurred to him after finishing his Gospel. It does not settle the question that Acts obviously harkens back to the themes and style of Luke, because so would an unanticipated sequel or the second volume of a two volume work. There are, however, features of Luke which indicate that its author all along planned on writing a sequel.
First, the two prefaces to Luke-Acts indicate that Acts is the second volume of a two-volume literary effort, not just a sequel.
Acts begins with a ‘secondary prologue,’ a device used for introducing new segments to works consisting of more than one book. Luke’s, of course, was a two-volume work: and Luke 1:1-4 is the ‘primary preface’ for his entire work, including Acts. In Hellenistic literature a secondary preface usually consisted of a brief summary of the prior volume followed by a short introduction to the matter to be covered in the new volume.
John Polhill, Acts, The New American Commentary, page 78. Notably, the second preface does not summarize its content because it is continuation of the subject matter described in the first preface. F.F. Bruce, The Acts of the Apostles, page 98 (“As the Gospel records what Jesus began to do and teach (cf. Luke 3:23), so Acts records what he continued to do and teach, by his Spirit in the apostles, after he was ‘taken up.’”).
Second, the contents of the preface also indicate that Acts is a second volume rather than merely a sequel. It is interesting that the author refers to the “things accomplished among us” in the preface to his Gospel.
It is, then significant that Luke refers to the ‘things’ which have ‘come to fulfilment’ among ‘us’. Granted that the ‘us’ refers to Christians generally, the phrase is more easily explained if it refers to what happened in the experience of the readers and therefore includes the growth and establishment of the Christian churches. The use of ‘things’ in the plural is an odd way of referring simply to the life-story of one person. And the use of ‘fulfil’ may also suggest more than simply the life of Jesus, the more especially since Jesus himself spoke of things that were yet to be fulfilled in the activity of his followers (Lk. 24:47-9).
I. Howard Marshall, “Acts and the Former Treatise,” in The Book of Acts in its Ancient Literary Setting, page 173.
Third, the author “saved” material for Acts that he could have used in his Gospel. Here, I will discuss two examples. In Jesus’ trial before the Sanhedrin Luke does not emphasize the “false witness” regarding the destruction of the Temple that is prominent in the Gospel of Mark. (Mark 15:57-59, compare with Luke 22:64-67). Obviously, the author is aware of the tradition of false witnesses claiming Jesus threatened the Temple. But he chooses not to emphasize it in the Gospel. Instead, he saves it for Acts. During the arrest of Stephen in Acts, “false witnesses” claim Stephen spoke against the Temple at the behest of Jesus. Acts 6:14.
Another example is Mark’s discussion of the Torah and purity in Mark 7:1-23 (“That which proceeds out of the man, that is what defiles the man.” v. 20). Once again the author does not include this in his Gospel, most likely because he highlights the suspension of the purity laws through Peter’s vision in Acts 10. (“What God has made clean, you must not call profane.” v. 15). It seems clear, therefore, that while writing his Gospel, the author consciously chose to save some material and/or themes for use in the anticipated second volume—Acts. As Daniel Marguerat notes, such examples shows “that the narrator envisions a plot whose completion in Acts 28 rather than Luke 24.” The First Christian Historian, page 48.
Fourth, the author of Luke also added material to his Gospel to set up material he placed in Acts. An example of this is the author’s use of “to all the rest” and “and those who were with them” in the Gospel (24:9 and 24:33 respectively). The only time these phrases are used in the Gospel is when the author is referring to “the eleven,” that is, the Twelve disciples minus Judas. Importantly, these verses deal with the eleven and “the rest” witnessing the resurrected Jesus.
In “The Eleven and Those With Them According to Luke,” Joseph Plvenik convincingly demonstrates that these two phrases are the result of Lukan redaction. CBQ, pages 205-11 (1978). But why emphasize that the “eleven” had company on these occasions? The answer is found in Acts, where the author makes his only other reference to the “eleven” (as meaning the Twelve minus one). In Acts 1:26, the “eleven” were brought up to “twelve” again by the drawing of lots and addition of Matthias to their number. Of course, for the author, an apostle had to be a witness to the resurrection of Jesus. “The expressions ‘the rest’ and ‘those with them’ thus reflect the traditional concept of apostleship described in Acts 2:32; 3:15; 4:33; 13:15, which echoes 1 Cor. 9:1-2.” Ibid, page 209. So, the author made sure that readers of his Gospel would know that there were other followers of Jesus present at his resurrection who would be qualified to assume the office of Apostle. Accordingly, there is evidence that the author added references to the Gospel that he was already planning to build on when writing the Acts of the Apostles.
Finally, it appears that the end of the Gospel is specifically intended to set the board for the Acts of the Apostles:
[T]he conclusion to Luke’s Gospel provides an introduction to the Book of Act. Jesus’ final words to his disciples are a virtual summary of the main themes of the first chapters of Acts—the waiting in Jerusalem until clothed with the power of the Spirit, the preaching to all the nations beginning with Jerusalem, and the fulfillment of the Scriptures in the death and resurrection of the Messiah, which is the central topic of Peter’s sermons in Jerusalem (Luke 24:44-49). Then there is the ascension. In all the New Testament the ascension narrative is related only in Luke and Acts, though several passages in the epistles refer to Jesus seated at God’s right hand (e.g. Heb. 1:3). It closes the Gospel of Luke and opens the Acts of the Apostles, binding Luke’s two volumes together.
Polhill, op. cit., page 24.
Taken as a whole, the evidence is convincing that the author wrote his Gospel with the writing of Acts specifically in mind. Acts is not just a sequel or an after thought; it is the second volume of a two volume work. Does this point matter? Yes, it should be considered in determining the genre of Luke-Acts, it informs us that Acts must be read with the preface of the Gospel in mind, it has obvious implications for understanding some of Luke’s redacting activities, it suggests that the dating of the composition of the two volumes are not far apart, and it reminds us of the importance of reading both volumes with the other in mind.