Why were there Jews from many different countries in Jerusalem at Pentecost?
Jews from across the then-known world—from Great Britain to India, North Africa, Greece, and Asia Minor—came to Jerusalem because Pentecost was one of the seven Jewish feast days. These Jews were still practicing the feast days, and Pentecost was the fiftieth day after Passover.
Les explains that the setting is Acts chapter 2. Christ had just been crucified at Passover, then He was with them forty days in His post-resurrection ministry, and they waited another ten days until the fiftieth day, Pentecost. Every one of the seven Jewish feast days brought Jews to Jerusalem from the whole then-known world, predominantly the Roman Empire, from Great Britain to India, and from Northern Africa across the Mediterranean to Greece and Asia Minor. The Temple was still operating; no one had told these Jewish believers or anyone else to cast aside Temple worship or the Law. They were still practicing the seven feast days, and here was Pentecost. On that day they were all with one accord in one place, and suddenly there came a sound from heaven as of a rushing mighty wind that filled the house where they were sitting.