Why did Peter still consider it unlawful to associate with Gentiles if he was set free?
Peter still considered it unlawful to associate with Gentiles because he had not been set free from the Law. He remained a Law-keeping Jew, still bound to kosher rules and legalistic separation, even though he had recognized Christ as Messiah. His freedom from that separation came only through God's later direction.
Les explains that Peter was still a Law-keeping Jew, so he had not been set free from the Law's requirements. In the vision of the sheet with unclean creatures, the Lord told him, "Rise Peter, kill and eat," but Peter refused, saying he had never eaten anything unclean. That was because he had to keep kosher. Later, when Peter went to Cornelius's house, he still moved with trepidation; Les pictures the Lord pushing him some 80–90 miles up the coast, and Peter did not want to go any more than Jonah did. At Cornelius's door Peter said it was unlawful for him, a Jew, to keep company with a man of another nation. He said that because he was not yet free from the Law. The same kind of believing Jews under Peter's control in Jerusalem also maintained separation, were still temple worshipers, and remained legalistic Jews, though they had recognized Christ as the Messiah.