Why were the Jerusalem believers still law-minded after Pentecost?
They remained law-minded because God had not yet revealed the secret of grace going to the Gentiles; He was still dealing with the nation of Israel under the law. Even years after Pentecost, believing Jews argued that Peter should not enter or eat with uncircumcised Gentiles, reflecting a continuing law-keeping mentality.
Les explains that when Peter went up to Jerusalem, the believing Jews of the circumcision contended with him. They objected that he went in to men uncircumcised and ate with them. Their mentality was still law: you cannot eat Gentile food because it is not kosher, and you cannot visit Gentiles, who are considered uncircumcised dogs. This was still the mentality of the Jerusalem believers seven or eight years after Pentecost. The reason was that God had not yet revealed the secret of grace going to the Gentiles; He was still dealing with the nation of Israel under the law. Les also cites Acts 22 and Ananias, a devout man after the law, to show that even Ananias was a law-keeping Jew. Peter, likewise, was not aware that God was going to Gentiles.