Probably the strongest evidence in Acts that Paul was a Torah-observant Christian is found in Acts 21:17–26. The pertinent facts of this episode are: (1) Paul arrives in Jerusalem and reports about his ministry among the Gentiles (v. vv. 17–19), (2) Paul’s report is received with praise to God (v. 20a), (3) Paul is informed about the growth of the Jerusalem church and that he has been accused of teaching the Jews living among the Gentiles to forego the Law and other Jewish customs (vv. 20b–21), (4) James and the elders of the church in Jerusalem encourage Paul to demonstrate his commitment to the Law by sponsoring and participating with four other Jewish men in certain purification rites (vv. 22–24b), (5) the stated purpose of this participation would be to refute the accusations made against Paul (v. 24c), (6) James and the elders of the Jerusalem church reaffirm their commitment to the agreement reached at the Jerusalem Council concerning Gentile Christians (v. 25; cf. 15:20, 29), (7) Paul complies with the suggestion of James and the elders (v. 26).
I want to make four points and then offer a tentative conclusion. First, it is absolutely essential to understand that the stated controversy in 15:20b–21 relates to whether or not Jews living among the Gentiles are required to keep the Law. Thus, the primary issue is not about the Gentiles and the Law which appears to have been settled in Acts 15 (21:25).
Second, it is worth noting that Paul appeared to be vulnerable to the accusations. So it is legitimate to ask, “If Paul was blatantly pro-Law, then how could the accusations have gained so much traction?”
Third, Paul’s actions are puzzling. On the one hand, Paul does not refute the accusations nor offer any explanation for them. On the other hand, Paul does agree to participate in James and the elders’ plan. We can assume that the accusations made against Paul were either true or false. If they were false,  Paul may have chosen not to defend or explain himself against what he might have seen as unfounded accusations unworthy of a response. But if this were the case, why would Paul agree to participate in James and the elders’ plan to refute what he might consider baseless charges? If the charges were true, or partly true,  Paul may have kept silent because he could not deny them. If the charges were true, Paul may have agreed to participate in James and the elders’ plan in order to soften or alleviate the criticism or to seek to maintain good relations with the Jerusalem church.  That Paul agreed to participate in the plan of James and the elders in order to maintain good relations and not for any a priori commitment to the Law seems likely since Paul never appeals to his participation in the plan during any of his defense speeches (22:1–21; 23:1–8; 24:10–21; 25:8–11; 26:1–29).
Fourth, the effectiveness of Paul’s participation in James and the elders’ plan is uncertain. Perhaps it would have helped, but one can only speculate based on Paul’s arrest at the temple on an unrelated charge. It is striking that Luke did not record any support from the Jerusalem church during Paul’s legal proceedings or incarceration. Paul appears to be strikingly alone. This may suggest that Paul’s attempt to address the concerns of his critics was not ultimately effective.
As I have noted in my previous posts of this series, the Paul of Acts was probably Torah-observant but the relationship of Paul to the Law is complicated. It is a legitimate question as to whether Paul’s relationship to the Law was driven by theological conviction (Paul as a Jewish Christian), ethnic identification (Paul as Jewish Christian), or evangelistic necessity (Paul as a Jewish Christian Evangelist).
 C. K. Barrett states that “Luke appears to assume that Paul did not do what he was alleged to do; the charge was not believed by the elders, it was false, and Paul will proceed by his actions to demonstrate his innocence” (C. K. Barrett, A Critical and Exegetical Commentary on the Acts of the Apostles: Preliminary Introduction and Commentary on Acts 15–28, vol. 2, ICC, ed. J. A. Emerton, C. E. B. Cranfield, and G. N. Stanton [Edinburgh: T & T Clark, 1998], 1007). In the quote above, Barrett apparently bases his argument on three points, all of which are questionable, especially the second “it was false.” This argument is circular because the point has not been demonstrated: what evidence does he draw from to show that the charge was not believed by the elders (they reacted to it), was false (Paul doesn’t say one way or the other), and that Paul was acting to prove his innocence (we don’t know if he is innocent).
 John Polhill argues that Paul was Torah-observant, but admits that “there may have been a grain of truth in the rumor that Paul was encouraging Jews of the Diaspora to abandon the Torah” in that the social situation of Paul’s primarily Gentile churches may have indirectly led some Jews to abandon Torah-observance (John B. Polhill, Acts, NAC, ed. David S. Dockery [Nashville: Broadman Press, 1992], 448). In a discussion on James 21, William Varner suggests that “Paul probably thought that such action was not necessary, but out of deference to James, he does it” (William Varner, The Book of James: A Linguistic Commentary Applying Discourse Analysis [Woodlands, TX: Kress Biblical Resources, 2010], 203). Varner does not provide any evidence to substantiate his point but sees in Paul’s response an application of the Pauline principle of “becoming all things to all men” (1 Cor 9:22).
 Neusner argues that James’ plan would have sent an ambiguous message and likely received a mixed response (Jacob Neusner, “Vow-Taking, the Nazirites, and the Law: Does James' Advice to Paul Accord with Halakhah?,” in James the Just and Christian Origins, ed. Bruce Chilton and Craig A. Evans, Supplements to Novum Testamentum 98 [Leiden: Brill, 1999], 81–2).