‘And when Peter was come up to Jerusalem, those who were of the circumcision contended with him, saying, “You went in to men uncircumcised, and did eat with them.” '
So when Peter arrived back in Jerusalem ‘those of the circumcision' came to him to ‘contend' (or ‘make a distinction') with him. In Acts 10:45 ‘those of the circumcision' had referred to Peter and the six men who were with him. It had meant simply all those who were present who were circumcised. We are not therefore probably intended at this stage to see it here as referring to a particular group. We may rather see it as referring to all who were there who were circumcised, (and so everyone), and as being used by Luke simply in order to emphasis their circumcision and contrast them with the uncircumcised whose position they were discussing. On this interpretation all the Apostles and brethren are thus to be seen as included in the description.
However, some see it as referring to a group who particularly stressed the need for circumcision and considered it a major issue. There would certainly be such a group later when circumcision had become an issue. And even now it may be that of those who had come to question Peter some (such as the Apostles) were neutral, waiting to hear his explanation, while others were specifically intent on taking up the issue of circumcision, a subject that they saw as of deep concern (although if that were so it is interesting that they did not do so). There would certainly be shades of feeling on how important the issue was, and on how important ‘cleanness' was. Not all the Apostles had always been too particular (Mark 7:2). And even Jesus would refrain from ritual washing in order to make a point (Luke 11:38). In the end it does not really matter, for all were undoubtedly there wanting to hear his explanation, and that was so whether they were included in the group or not.
As we look at the incident it is important that we recognise that this questioning of Peter was a valid and Scriptural procedure. The Old Testament made it incumbent on God's people to check out any instance where it appeared that God's Law had been broken (Deuteronomy 13:14), and it was right that no exception be made for Peter. Thus the enquiry is to be seen as having been a necessity, not an example of lack of trust or of love. From that point of view the important issue was not the enquiry, it was the attitude with which it was being conducted.
They ‘contended with him', ‘making a distinction' between him and them. The reaction was natural. It was not necessarily belligerent. It was the same way in which Peter would have reacted had he not had the vision that he had. They all wanted to know on what grounds he had behaved as he had by joining with the uncircumcised in their home and eating with them. Why was he ignoring the plain requirements of the Law (as they interpreted them)? They had nothing against him preaching to Gentiles in order to turn them into proselytes, but it was quite another to have close fraternisation with them, and to enter their homes and eat with them, homes where any kind of ‘uncleanness' may be hidden, and where the food would not necessarily be properly prepared and may have included ‘unclean' elements.
Yet this very questioning was good, for now they would have to square up to the answers. From these they would then have to determine their own position on the matter, and come to a verdict accordingly. They would either finish up by accepting Peter's new position and taking it for themselves, or they would harden their hearts and resist God's truth. (Those who did the latter would later form a circumcision party).