What does Acts 13:48 mean?

In Jesus' time and after, Gentiles had their pick of idols to worship. In Pisidian Antioch, home to numerous military members, the emperor cult was especially popular. But Gentiles often grew tired of trying to placate statues and sought out esoteric religions in the east to find meaning. Many of them looked to Judaism. Some, like Sergius Paulus, the proconsul of Cyprus, were fooled by the mysticism of false Jewish prophets (Acts 13:6–8). Others, often called "God-fearers," took the more traditional route of attending the Jewish synagogue and learning as the Jews did.

For a Gentile, entrance into full Judaism wasn't easy. They had to perform a ritual involving a sacrifice and ceremonial bathing, and the men had to be circumcised. Although the term "God-fearer" is used unevenly in the New Testament, it's safe to assume most Gentiles identified as such were not full converts. Their acceptance into the group known as "God's people" was limited.

Jesus is the Jewish Messiah, promised by God to come from the line of David (Jeremiah 23:5–6). The Jewish leaders in Pisidian Antioch had all the prophecies they needed to understand this, but they were jealous of Paul and Barnabas' popularity and wouldn't listen to their message. Some of the Jews did listen, and a great number of Gentiles. It is suddenly clear to the Gentiles that the God of the Jews is for them. The Savior that God promised the Jews is for them. Salvation is for them (Acts 13:47). The Jewish leaders, who should have accepted Jesus as their Messiah and invited the Gentiles into salvation, get jealous, instead.

As Paul's ministry expands, he will have to deal with this rivalry numerous times. In fact, when he and Barnabas return to their home base of Syrian Antioch, they will find that legalistic Jews have infiltrated the church there and demanded that Gentiles adhere to circumcision to follow the Jewish God. In order to have the authority for their convictions, Paul and Barnabas will bring their concerns to the first church in Jerusalem where Peter and James, the half-brother of Jesus, will confer with the other elders (Acts 15). The Gentiles here understand what will later become official church policy: God accepts them as Gentiles, and salvation is theirs.

Luke, here, makes a statement about predestination. Some of the Jews and Gentiles were "appointed" or chosen to be saved, and it is those who believed (see also Romans 8:30; Ephesians 1:3–6, 11). This shouldn't be hard to accept. Anyone reading this can be saved. In order to know if you are chosen, just believe (John 3:16–18; Romans 10:9). If you believe, you know you were chosen.