11 When Cephas came to Antioch, I opposed him to his face, because he stood condemned. 12 For before certain people came from James, he used to eat with the Gentiles. But when they arrived, he began to draw back and separate himself from the Gentiles because he was afraid of those who belonged to the circumcision group. 13 The other Jews joined him in his hypocrisy, so that by their hypocrisy even Barnabas was led astray.There's an important issue that we need to wrestle with in this passage, and it's the question of whether or not the people from James and the circumcision group are the same group. I am not inclined to think that they are. The ensuing discussion is drawn from Longenecker's commentary pp 73-5.
14 When I saw that they were not acting in line with the truth of the gospel, I said to Cephas in front of them all, "You are a Jew, yet you live like a Gentile and not like a Jew. How is it, then, that you force Gentiles to follow Jewish customs? (TNIV)
In verse 12, we are told of Peter's fear of 'the circumcision group.' There are three main options for who this 'circumcision group' is. They could be (1) Jewish Christians who claim that circumcision is necessary for Gentiles, they could be (2) Jewish Christians generally, or they could be (3) Jews generally. The Greek behind 'circumcision group' more literally is 'the circumcised ones.' The TNIV's wording shows that its team of translators opted for number 1. Longenecker, however, has convinced me that option 3 is actually the best choice.
There are two major reasons that he gives that I think together are somewhat decisive. One, is that when Paul uses the term 'circumcised' in the prior section, he's referring to Jews generally. This is in keeping with Paul's typical usage of this term. Second is that there seems to be no conceivable reason for Peter to fear a group of Jews when he was one of the leaders of the Jewish church and had been open in fraternization with Gentiles (see Acts 11).
On the other hand, we can suggest a plausible reason why there may be fear of the Jews at large. In the 40s and 50s there was a 'rising tide of Jewish nationalism in Palestine and its growing antagonism towards any Jew who had Gentile sympathies or who associated with Gentile sympathizers' (p. 74). To take Longenecker's point and run with it, it's very possible that Jews back in Jerusalem were getting increasingly intolerant with the Jerusalem church (remember, Christians were probably still involved in synagogues and the temple at this time). This meant that the mission to the circumcised was probably getting more difficult (as Longenecker points out) and it may be that there was growing persecution. Thus the church in Jerusalem and subsequently the Jewish Christians in Antioch may have taken a pragmatic approach for the time being. They knew that Gentiles didn't have to become circumcised, but for the sake of the gospel mission, and/or out of love for believers in Jerusalem, they may have pushed for Gentiles to follow Jewish dietary laws and get circumcised even though they knew that it wasn't required in an absolute sense. This seems to me to be the type of extenuating circumstance that could lead Peter to act against his earlier agreement (and act against what he believed as the term hypocrisy suggests he did).