“Neither went I up to Jerusalem to them which were apostles before me; but I went into Arabia, and returned again unto Damascus. Then after three years I went up to Jerusalem to see Peter, and abode with him fifteen days. But other of the apostles saw I none, save James the Lord's brother.” Galatians 1:17–19
Context is important, in verse 18 Paul says he was visiting with Peter, and more broadly, Paul met and had relationships with many Apostles, the Greek word for “apostle” literally means “one who is sent” and can refer to an emissary or anyone sent on a mission. An apostle is given the authority of the one who sent him. All of the apostles were disciples—they were among the many believers in Jesus—but only a select group of disciples were chosen as the Twelve Apostles (Matthew 10:1–4; Mark 3:14; Acts 26:14–18). This included the original twelve disciples (although Judas Iscariot eventually reversed his loyalties and rejected Christ) and includes Paul and Matthias. That there is a select group of twelve apostles is seen in the foundations of the walls of the New Jerusalem: twelve foundations, each inscribed with a name of an apostle (Revelation 21:14).
Other men who are named “apostles” in the New Testament—although not members of the Twelve—include Matthias (Acts 1:26), Barnabas (Acts 14:14), Apollos (1 Corinthians 4:6–9), Timothy and Silas (1 Thessalonians 1:1, 2:6), Epaphroditus (Philippians 2:25), and two unnamed apostles (2 Corinthians 8:23, possibly already included in the previous list). These men were “sent ones” in that they were chosen for specific work on behalf of the church, but they were not part of the Twelve who were hand-picked by Jesus. Jesus is also called an “apostle” in Hebrews 3:1, indicating that He was sent by and had the authority of His Father.
As the early church began to grow, in spite of Herod’s persecution, Barnabas was called by the Holy Spirit to go with Paul on a missionary journey. Barnabas’ cousin, John Mark, served him and Paul as their assistant (Acts 13:5). During that first missions trip, for an unspecified reason, John Mark left them and did not complete the journey (Acts 13:13). However, Barnabas continued with Paul and was with him when Paul’s ministry was redirected to reaching the Gentiles with the gospel (Acts 13:42-52). The only negative mention of Barnabas in Scripture is in reference to an incident in which Peter’s hypocrisy influenced other Jews (including Barnabas) to shun some Gentiles at dinner (Galatians 2:13).
Paul and Silas ministered together on the second missionary journey (Acts 15–18). Paul and Silas are first mentioned together after the Jerusalem Council in Acts 15, as both men were part of the group that took the council’s decision back to Syrian Antioch (verse 22). Silas is called a “prophet” who “said much to encourage and strengthen the believers” in Antioch (verse 32).
Timothy, the recipient of the two New Testament letters bearing his name, was the son of a Greek father and a Jewish mother. He joined Paul during one of Paul’s later missionary journeys. Paul addresses Timothy as "my true son in the faith" (1 Timothy 1:2). He was probably no older than late teens/early twenties when he joined Paul but had already distinguished himself as faithful, and the elders noticed him. He probably heard and responded to the gospel when Paul came through the area of Derbe and Lystra on his first missionary journey, but we don’t know for sure. Timothy served as Paul’s representative to several churches (1 Corinthians 4:17; Philippians 2:19), and he was later a pastor in Ephesus (1 Timothy 1:3). Timothy is also mentioned as being with Paul when Paul wrote several New Testament letters—2 Corinthians, Philippians, Colossians, 1 and 2 Thessalonians, and Philemon.
Paul ministered with Titus as well….”Then fourteen years after I went up again to Jerusalem with Barnabas, and took Titus with me also.” Galatians 2:1
Paul also met with Peter, James and John..
“But contrariwise when they saw that the gospel of the uncircumcision was committed unto me, as the gospel of the circumcision was unto Peter; (For he that wrought effectually in Peter to the apostleship of the circumcision, the same was mighty in me toward the Gentiles:) And when James, Cephas, and John, who seemed to be pillars, perceived the grace that was given unto me, they gave to me and Barnabas the right hands of fellowship; that we should go unto the heathen, and they unto the circumcision. Only they would that we should remember the poor; the same which I also was forward to do. But when Peter was come to Antioch, I withstood him to the face, because he was to be blamed. For before that certain came from James, he did eat with the Gentiles: but when they were come, he withdrew and separated himself, fearing them which were of the circumcision. And the other Jews dissembled likewise with him; insomuch that Barnabas also was carried away with their dissimulation. But when I saw that they walked not uprightly according to the truth of the gospel, I said unto Peter before them all, If thou, being a Jew, livest after the manner of Gentiles, and not as do the Jews, why compellest thou the Gentiles to live as do the Jews? We who are Jews by nature, and not sinners of the Gentiles” Galatians 2:7–15