What does Acts 10:45 mean?
Peter had just begun telling a roomful of God-fearing Gentiles about Jesus' role as the judge of the world when the Holy Spirit falls on them (Acts 10:30–44). Two days earlier, in the town of Joppa by modern-day Tel Aviv, he had received a vision that softened him to the idea that Gentiles could follow Jesus (Acts 10:9–16). Six Jewish Jesus-followers from Joppa came with him when messengers arrived and requested he come to Caesarea Maritima to speak with a Roman centurion (Acts 10:17–23; 11:12). Now the centurion and his friends and family are Jesus-followers.Peter is witnessing the Holy Spirit indwell the Gentiles. When he returns to Jerusalem, he will testify to what happened with the six to validate (Acts 11:1–18). Later, he will again confirm that these people are God-fearing Gentiles but not full proselytes—the men are not circumcised (Acts 15:7–9).
The "circumcised" mentioned here are the six men from Joppa; the term is used to refer to Jews. Later, Paul will talk of the "circumcision party" in a derogatory way to mean Jewish Jesus-followers who want to force Gentiles to follow the Mosaic law. Their interference will cause Paul and Barnabas to travel to Jerusalem to get clarification on the requirements for Gentiles (Acts 15:1–5). Men from "the circumcision party" will also temporarily convince Peter and Barnabas to stop eating with Gentile Jesus-followers, much to Paul's frustration (Galatians 2:11–14). Paul will call these legalistic Jews "those who mutilate the flesh" (Philippians 3:2) and tell the Galatians that they should just go all the way and castrate themselves (Galatians 5:12).
That isn't to say the medical/social practice of circumcision is bad. It just isn't required to be a Christian. The wider argument over Christian circumcision hadn't started at this time. Luke—the author of this book—is just distinguishing the new Gentile Jesus-followers, who aren't circumcised, with the Jews, who are.
Acts 10:44–48 records the start of the international church. Peter is in Caesarea Maritima, sharing the story of Jesus with a houseful of Gentile God-followers. Before he is even finished presenting the gospel, the Holy Spirit falls on them. This is the only occasion in Acts where the Holy Spirit comes on people before they were baptized. This will rock the Jewish church to its core and prepare the way for Paul's ministry (Acts 11).
Peter has been a dominant voice in the spread of Jesus' message to Jews and proselytes. Now he brings the gospel to Gentiles. An angel tells Cornelius, a centurion, to ask Peter to come to him. Peter is praying when he receives a vision of food—including non-kosher food—and God's voice telling him to eat. When the centurion's messengers arrive, Peter realizes the dream meant that Gentiles are no longer unclean. He follows the messengers and tells Cornelius' household about salvation through Jesus. Before Peter can lay his hands on them or baptize them, the Holy Spirit falls on them.