When did the earliest believers begin to question the “boundary markers” of Judaism? By “boundary markers” I mean primarily circumcision, food laws and keeping Sabbath. It is not really possible to describe Peter and John as preaching to Jews in the Temple that what Jesus did on the cross freed them from the Law. One reason for this is that there were few Jews who saw the Law as a slave master from which they longed to be free. For the men worshiping in the Temple, and likely for those in the Greek-Speaking Synagogue of the Freedmen, keeping the law was a privilege given to them by God. There were likely few Jews if any who would have relished the chance to throw off the constraints of the Law. In fact, the Maccabean Revolt indicates that the majority of Jews were willing to fight in order to be allowed to keep the Law!
For me, this indicates that the Jewish believers in Jerusalem continued to practice Judaism in every way. The question “should we continue to circumcise our children” or “should we eat prohibited foods” simply would never have come up in the early years. Jesus is Messiah and Savior, but he did nothing to cancel the Jewish believer’s commitment to the Law. Another indication of this is that many Pharisees and other “zealous” Jews joined the Jerusalem church (Acts 15:5, 21:20-21). If Peter, John, Stephen or Philip urged Jews to defect from the Law, the reaction to Paul is unintelligible.
The boundary markers only became and issue after a significant number of Gentiles joined the church, likely in Antioch first, but certainly in Paul’s first churches in Galatia. Acts 11:20 indicates that the church at Antioch limited their evangelism to Jews until men from Cyprus came and evangelized the Hellenists. The noun Eλληνιστής refers to Greek speaking Jews (BDAG), not Greeks. The ESV footnote says that the word refers to Greek speaking non-Jews, but this explanation is not correct and misses the point Luke is trying to make. The Christians at Antioch are targeting both Hebrew/Aramaic speaking and Greek speaking Jews just like what was happening in Jerusalem until the persecution scattered the believers.
Even if these Hellenists are Gentiles, it is likely that the Gentiles who were joining the church in Antioch were doing so as God-fearers. This was the recognized practice in the synagogues anyway. There was no compulsion for these God-fearing Gentiles to submit to circumcision, although it appears that in every other respect they kept the Law and traditions of the Jewish people. The fact that the apostolic representative Barnabas was pleased with the progress in Antioch indicates that the Law is still respected and kept in these Christian synagogues.
So there is really no “questioning of the boundary markers” until the first Pauline mission, when the gospel is preached outside of the synagogue and Gentiles who were not already God-fearers accepted Jesus as savior. If the story ended in Acts 11, then Christianity would have been a sect of Judaism.