Peter had been faithful to the law in his manner of life as was evident in his response when God first spoke to him regarding the Gentiles in Acts Chapter 10. He was known in Jerusalem as an apostle to the circumcision and must have been a strong proponent of adherence to the law after salvation as he led the church in Jerusalem after the day of Pentecost. It was about eight years after Pentecost that God sent him to the Gentiles. AND it was now more than fifteen years after his visit to the Gentiles and the Judean believers were still practicing the law of Moses. Those on the side of circumcision no doubt expected that Peter would back "circumcision before salvation" and they had most likely forgotten that God first sent Peter, not Paul, to bring His Word to the Gentiles. However, Peter had not forgotten the grace of God he had witnessed at the house of Cornelius.
Peter was very respected in Jerusalem and he had in fact led the first century church for many years (although there is evidence in the book of Acts that James the half-brother of Jesus Christ was at this time in the chief position.) In his presentation he made several important points:
1) God had given Cornelius and his household the gift of holy spirit without requiring circumcision, conversion to Judaism, or adherence to the law of Moses. Only faith [believing] in Christ was required.
2) Neither Judeans nor Gentiles are able to fulfill the law. To require the Gentiles to do so would be to require something that the Judeans had not been able to do.
3) Both Judeans and Gentiles receive salvation the same way - BY THE GRACE of the Lord Jesus Christ - not by works.
Peter's presentation set the stage for the council to hear from Paul and Barnabas about the gospel of the grace of God in action among the Gentiles.
Gladly
Acts