a centurion: This was the highest rank for an officer in the Roman army who was not commissioned. He led a century, a group of about 100 men. Luke often describes centurions positively (10:22; 21:32; 22:25–26; 23:17, 23; 27:6, 11, 43; 28:16; Luke 7:1–10; 23:47).
Luke wanted to show that Christianity was not against Roman officials or institutions. It could, like Judaism, be permitted in the Roman Empire (see Acts Book Introduction, “Purposes of Acts: Politics”).
in what was called the Italian Regiment: A regiment included six centuries; a Roman legion was usually divided into ten regiments. The New Testament mentions the Italian Regiment and the Imperial Regiment (Acts 27:1).