David W. answered 05/09/16
Experienced Prof
The 12 disciples of Jesus (minus Judas, who betrayed Him) became Jesus’s apostles. In Acts chapter 1, Matthias was selected to join the eleven, now called apostles (note the qualifications in Acts 1 for an apostle).
Paul (formerly Saul, the fierce persecutor of Christ-followers) was introduced to the church in Jerusalem by Barnabas (Acts chapter 9). Paul preached boldly. He was now accepted by them.
When the believers in Jerusalem heard about non-Jews being saved in Antioch, they sent Barnabas there. When Barnabas saw how God was working, he went back to Jerusalem to get Paul to help him preach and teach. The two of them spent two years in Antioch and the believers were first called Christians in Antioch (Acts chapter 11).
The church at Antioch sent Barnabas and Paul on a missionary journey. A few years later, Paul took Silas on a second missionary journey. By this time, he was widely recognized as an apostle by Christians everywhere.
Paul wrote letters to the churches and to individuals with the salutation of “Paul, an apostle …” (see Romans, 1st and 2nd Corinthians, Galatians, Ephesians, 1st and 2nd Timothy and Titus) as well as defending his apostleship in 1st Corinthians chapter 9 and in 2nd Corinthians chapter 12.
Paul had spent time in Jerusalem, so the apostles knew him well. He wrote letters that were circulated to the various churches, so he was widely known as “The Apostle Paul” by the time that he became a martyr in Rome. In 2nd Peter 3:15, Peter, a disciple and one of the Jerusalem apostles, calls him, “beloved brother Paul.”