St. Barnabas, Apostle to the Gentiles, has traditionally been regarded as an equal to the original twelve disciples. A Jew and a Levite, he was born on Cyprus, but later moved to Jerusalem (Acts 4:36-37) and converted to Christianity in about 29 or 30 A.D. After St. Paul's own dramatic conversion from persecutor to adherent of the Faith, St. Barnabas persuaded the understandably suspicious disciples in Jerusalem to receive him (Acts 9:27). Later, St. Barnabas was sent by the Church in Jerusalem to inquire into the fact that Gentiles in Antioch had heard the Gospel and were being converted. What Barnabas discovered in Antioch convinced him that the grace of God was at work among the Gentiles of that city, and he recruited St. Paul to join him in giving these new converts proper instruction in the Faith. The work prospered there, and Antioch became a base from which St. Barnabas and St. Paul conducted a number of missionary journeys. At the Council of Jerusalem, their work among the Gentiles was endorsed by the original disciples and the mother church (Acts 14:27-15:30). Of the later work of St. Barnabas, little is known for certain--there being a number of traditions--but he was certainly among the most eminent men of the Church's first century.
For more information, see the article by John F. Fenlon in The Catholic Encyclopedia from which this brief account is abstracted.