"The place of chap. 15 in the structure of Acts is difficult to determine. Historical knowledge exacerbates the question, since commentators know that, subsequent to the sequel of this meeting, Paul began his career as an independent gentile missionary. If, as has often been proposed, this is the central chapter in Acts, does it complete the first half, begin the second, or is a division into two principal parts undesirable? Although the chapter deals with the central issue of Acts—the legitimacy of the gentile mission—and occurs in the center of the book, it is not the basic structural pivot, nor does it break new ground. Acts 15 is central in that it brings together the various threads of the plot: Peter with his mission, Barnabas and Paul with theirs (which is the unacknowledged successor of the ‘Hellenists’), and those persons concerned with observance. The two geographical bases, Jerusalem and Antioch, also formally converge. The narrator wishes to make a profound impression with a grand assembly and a solemn decree. All of this is to paper over the rift between Paul and Peter, and the tension between Paul and James. When that tension finally emerges, in chap. 21, the reader is directed to perceive their encounter in the light of chap. 15."
Richard I. Pervo and Harold W. Attridge, Acts: A Commentary on the Book of Acts, Hermeneia–A Critical and Historical Commentary on the Bible (Minneapolis: Fortress Press, 2009), 367-8.
I tend to agree with much of what Pervo states above. But I am not sure that the argument of the last two sentences is sustainable. For one thing, how can you paper over a rift between Peter and Paul and then suggest that the tension emerges in chapter 21, when Peter is not even mentioned in chapter 21? Furthermore, even though James is mentioned in chapters 15 and 21, Acts 21 states that the tension is not so much between James and Paul, but between Paul and Jewish Christians who had been told that Paul was teaching Jews not to keep the Law (21:21). James actually plays a mediating role. Indeed, if there was such tension between James and Paul, why does James respond positively to Paul’s ministry report (21:20) and why would James offer Paul a way to appease the Law-observant Christians? One final point, a significant one, needs to be made. The issue in Acts 15 relates to Gentile Christians and the Law. The issue in Acts 21 relates to Jewish Christians and the Law. These are not the same issue. While James does reiterate the prohibitions from Acts 15 in Acts 21, the flow of James’ argument suggests that he understands the differences between the issues. The peri de that of 21:25 implies a change of topic, namely, that James is shifting the topic from Paul and the Jews to the Gentiles. All this is to say, neither Acts 15 or 21 picture a rift or tension between Paul and Peter and James. One might debate whether such a rift existed, but the evidence for such a rift is not to be found in Acts 15 and 21.