Paul returns to Jerusalem and is greeted warmly, but told that some unnamed members of the church had accused him of teaching "all the Jews living among the Gentiles to forsake Moses, and that you tell them not to circumcise thier children or observe the customs" (21). They recommend he go through the Nazarite vow (an ancient Jewish purification ritual, evidently), and tell him of the contents of the letter mentioned in Acts 15 regarding the rules for Gentiles.
Why are people accusing Paul of this? Telford mentioned this back in the same post I linked to at Acts 15:
Paul seems to take this conclusion as still too conservative, for he moves beyond Jerusalem's sphere of influence, concentrates his mission farther westward, and never mentions James' letter. Even when he mentions the council of Jerusalem's conclusion that circumcision of Gentiles is unwarranted (Gal. 2:1-10), he fails to mention these guidelines (unless Gal. 2:10's "remembber the poor" is some kind of extreme gloss). The Didache might reflect the council's guidelines, but only very vaguely: "As regards diet, keep the rules so far as you are able; only be careful to refuse anything that has been offered to an idol, for that is the worship of dead gods" (chapter 6). There is definitely variety in early Christian attitudes on these matters, and maybe even persistent tension. Weirdly, Acts 21:17-26 shows James receiving Paul and informing him of the letter as if Paul has not seen it before! (This might have something to do with Luke's use of a different source for that section of Acts, but I wonder whether in the final narrative it shows us relations that are still a little strained.)
Possibly it does, making it even stranger that Jesus just trusted that everybody would figure out what Jewish rules applied to Gentile converts. Makes my skeptical mind wonder if Jesus even knew that there would be Gentile converts, but that would be extra problematic...