Mark D. Nanos, Reading Paul within Judaism: Collected Essays of Mark D. Nanos, vol. 1 (Eugene, OR: Cascade Books, 2017), 24-25.
There is no reason to believe that Paul opposed circumcision of children born to Jewish parents, and good reason to suppose that he did not. And there is no reason to suppose that he opposed circumcision of non-Jews who were not Christ-followers. At many points the logic of his position suggests that Jewish believers in Christ, including Paul, observed his instruction to remain in the state in which they were called, keeping the commandments of God (1 Cor 7:17-24), which, for a Jewish person, involved guarding the whole Torah, by Paul’s own admission (Gal 2:15; 5:3; 6:13; discussed below). And it makes sense to suppose that Paul, like the Christ-following Jews described by James in Acts 21, would be zealous in his observation of halakhic behavior, and take the steps necessary to demonstrate this fact and dispel any rumors that he did not do so. The Luke that Paul presents, who undertakes a Nazarite vow in the temple, which involved a burnt offering (Acts 21:19-26) appears to signify a later interpreter presenting Paul similarly on this matter.