Why did Peter say it was unlawful for him to visit Cornelius?
Peter said it was unlawful because he was still a law-keeping Jew under the Law, which forbade a Jew to come unto one of another nation. The context explains these believing Jews had not been told to stop keeping the Law, so Peter viewed visiting Cornelius as a violation of that law.
Les teaches that in the Book of Acts, all these believing Jews who accepted Christ as the Messiah were still law-keepers. No one had told them to quit going to the Temple or stop keeping the Law. Even seven years after Pentecost, Ananias was a devout believer but still under the Law. So when Peter got to the door of Cornelius, he was still operating under that law-keeping mindset. He told Cornelius that it was an unlawful thing for him, a Jew, to come unto one of another nation. Later, when the sheet came down and the Lord said, Rise, Peter, kill and eat, Peter reacted, No way, Lord! I've never eaten anything that wasn't kosher. I've never eaten anything that was unclean. He hadn't because he was a law-keeper. This illustrates why Peter saw visiting Cornelius as unlawful.