Although St. Barnabas was not one of the Twelve, he is referred to as an "apostle" in the New Testament.
While Jesus named 12 apostles during his ministry, the title of “apostle” is often applied to other New Testament figures.
St. Paul is among the most widely known apostle, even though he was never associated with the Twelve until after Jesus’ death and resurrection.
St. Barnabas is another example of a New Testament figure who was given the title of “apostle.”
What’s interesting is that the author of the Acts of the Apostles gives both of these individuals this title, “the apostles Barnabas and Paul” (Acts 14:14).
The English word “apostle” is derived from the Greek apostello, “to send forth.” An apostle is one who is sent by God to preach the Gospel to the nations.
As a result, while Paul and Barnabas were not originally chosen by Jesus to be part of the Twelve, the Church believed that they shared in the same ministry, having been called by God to preach the Gospel.