All we know of Barnabas is to be found in the New Testament. A Jew born in Cyprus and named Joseph, he sold his property, gave the proceeds to the Apostles, who gave him the name Barnabas, and lived in common with the earliest converts to Christianity in Jerusalem. Originally a Levite, he is thought to have been a Hellenized Jew and to have been one of the Apostles of the 70 (or 72). He persuaded the community there to accept Paul as a disciple, was sent to Antioch to look into the community there, and brought Paul there from Tarsus. With Paul he brought Antioch's donation to the Jerusalem community during a famine, and returned to Antioch with John Mark, his cousin. He is said to have been among the founders of the church in Antioch in Pisidia. He introduced Paul to the apostles after his conversion. The three went on a missionary journey to Cyprus, Perge (when John Mark went to Jerusalem), and Antioch in Pisidia, where they were so violently opposed by the Jews that they decided to preach to the pagans. Then they went on to Iconium (Konya) and Lystra in Lycaonia, where they were first acclaimed gods and then stoned out of the city, and then returned to Antioch.
When a dispute arose regarding the observance of the Jewish rites, Saint Paul and Barnabas went to Jerusalem, where, at a council, it was decided that pagans did not have to be circumcised to be baptized. On their return to Antioch Barnabas wanted to take John Mark on another visitation to the cities where they had preached, but Paul objected because of John Mark's desertion of them in Perge. Paul and Barnabas parted, and Barnabas returned to Cyprus with Mark. Nothing further is heard of him, though it is believed his rift with Paul was ultimately healed. Tradition has Barnabas preaching in Alexandria and Rome, the founder of the Cypriote Church, the Bishop of Milan (which he was not), and has him stoned to death at Salamis about the year 61. The apocryphal Epistle of Barnabas was long attributed to him, but modern scholarship now attributes it to a Christian in Alexandria between the years 70 and 100. The Gospel of Barnabas is probably by an Italian Christian who became a Mohammedan; and the Acts of Barnabas once attributed to John Mark are now known to have been written in the fifth century. His feast day is June 11.
He is traditionally considered the founder of the Cypriot church and to have been martyred at Salamis c. 61. Some say he founded the See of Milan. Tertullian says that Barnabas, not Paul, wrote the epistle to the Hebrews, and The Epistle of Barnabas is of unknown authorship.
Sources: Bible and Christian websites