The shift of proclaiming the gospel to the gentiles has its first major push back over the issue of circumcision. A group commonly referred to as the circumcision party, also referred to as Pharisees in chapter 15 verse 5, begins teaching to the gentiles that have embraced Christ through faith that they must be circumcised. Paul and Barnabas initially argue with them, but it is wisely decided to resolve the dispute by travelling to Jerusalem and holding what becomes known as the Jerusalem Council. The decision in Jerusalem by a meeting of the elder leaders of authority, including James the brother of Jesus and Peter, is found in verse 9, "...having cleansed their hearts by faith," and verse 11, "...we believe that we will be saved through the grace of the Lord Jesus, just as they will."
The leaders from Jerusalem send out a list of four things to abstain from, and there is a reference to synagogues located in many different ancient cities. This seems to point to some practical considerations. We are not bound by the Mosaic covenant, but there are still many things we can and should learn from the old testament to more fully grasp our new covenant. Paul and Barnabas return with the leaders judgment regarding circumcision.
There is an intense argument that takes place upon their return between Paul and Barnabas about whether to take John Mark with them on a second missionary journey which causes them to part ways with Barnabas and John Mark going to Cyprus, and Paul and Silas going north through Syria and the eastern shore of the Mediterranean into modern day Turkey. This break causes me mixed feelings because it seems so tragic for such great friends and men of God to be divided by passionate disagreement, but in a strange way, it comforts me to know that they are just humans like us and that God can obviously use flawed humans in such a powerful way.
Anyhoo, Paul sets off with Silas and soon meets a young man named Timothy who becomes like a son to Paul (we see this in the letters to Timothy from Paul, first and second Timothy, found later in the Bible). They go off on more missionary journeys revisiting some cities and spreading the gospel to some new locations. Paul visits and stays in many of the cities he later writes to. These letters become preserved for our benefit. We see him in Philippi in Acts 16:12, in Thessalonica in 17:1, and in Galatia, which is an entire region rather than a city, in 16:6. We see Paul stay in Corinth for over a year and a half in chapter 18. He visits Ephesus more than once in chapters 18 and 19, and in Acts 20:31, he mentions having spent three years there. A more exhaustive look at all three of Paul's missionary journeys would no doubt be fruitful, but I am primarily going to simply point out that by the end of the 50's A.D., Paul and others have truly spread the gospel throughout the Roman empire and beyond. This can be seen not only through the narrative of Acts itself, but by how, in the later chapters, people react in ways that show they are aware of the disciples and Christ and Christians. Compare the Jewish leaders response to invite Paul to speak in the synagogue in Acts 13:15 to the response Paul gets from Jewish leaders in the Temple in Acts 21:28, "....Men of Israel, help! This is the man who is teaching everyone everywhere against the people(Jews) and the law and this place(Temple)..."
The final push of the gospel's reach "to the ends of the earth" begins with Paul and Barnabas in chapter 13 and is still going on today. Paul's "final missionary journey" will be with an armed guard and increasingly, his audience will be those with greater and greater authority.