Jesus had said to His disciples that the Gospel of sins forgiven had to be preached to all nations, beginning at Jerusalem (Luke 24:47). That was also why they had to wait for the Holy Spirit's baptism. He reminded them of John's baptism which had prepared the way for Jesus to pour out His blessing upon God's historic people. When the apostles were baptised with the Spirit's power, then all of Jesus' disciples would begin a process that had to take in the entire globe. It did not become global because they then happened to be living under the Roman Eagle. It was global. This Messiah proclaims God's Grace for all.
Luke's contribution in his Gospel and Acts is crucial for us now. We live with a strong legacy reaching back over two thousand years and we inherit traditions of law, social life and government from Roman civilisation. Acts helps us understand how it was that a group of small communities, following Jesus of Nazareth, became the salt of the earth, the leaven which leavens the entire lump. It tells how God's Spirit was poured out upon women and men. Luke's story is about the miracle of God's grace going forth to conquer far and wide. With the Spirit's leading the Book of Acts gives us decisive "hints" about how this work proceeded, even to this day. Luke helps us discern where we are in relation to the most recent phase of God's timetable. The entire earth and our human activities within it are being remade to serve within His Kingdom of Righteousness and Grace.
Luke is not primarily concerned to tell a story about how a new religion became a significant power - he assumes that those called "Christians" lived by the power of the Holy Spirit. And yes these "Christians" from the earliest times made Roman administrators scratch their heads. Luke's earliest reference to Rome in Acts is in the list of places from which the God-fearing, devout Jews had come to Jerusalem. It is almost off-hand - "and visitors from Rome, both Jews and proselytes" (Acts 2:10b). While non-Jews living in the Roman Empire are indeed "Gentiles", the term "Gentile" is not confined to those in the Roman Empire. "Gentiles" means "nations", a Jewish term for those other than themselves. And so, in the miraculous event on the day of Pentecost, it was God-fearing Jews "from every nation under heaven" - north, south, east and west - living amongst the gentiles - who had come together and heard the apostles. The phrase "under heaven" emphasises that they were the people of God's promise, believers, "under heaven", whether they lived in regions "under the Roman Eagle", or not. Of course it seems highly unlikely that Jews had gone to live with Australian aborigines or the South Pacific islanders although we do not know the exact details of the furthest reaches of their dispersion among the nations. But these converted Jews were God's way of preparing the soil so that the seed could take root back in their own communities, among the nations, unto the ends of the earth. They had heard about God's Grace in the gentile languages spoken back home. The people of these tongues, whoever they were, would come to hear that God, by raising Jesus from the dead, had conquered death. To this Lord and Christ all rulers, princes, kings and Caesars must pay homage.