Why is St. Paul called an apostle?  He wasn’t one of the twelve apostles that Jesus picked. 

The word “apostle” comes from the Greek “apostolein,” meaning “sent ones.” Although Jesus specially designated twelve of his followers in a symbolic restoration of the twelve tribes of Israel (see Matthew 10:2-5, Mark 3:16-19, and Luke 6:13-16), these twelve men were not the only ones sent by Jesus. Mary Magdalene and the other women who saw the risen Jesus were sent by him to share the good news of the resurrection with the other disciples; before the ascension all of the disciples were sent forth by Jesus to proclaim salvation to the ends of the earth. Paul, though not one of the original companions of Jesus, considered himself an apostle sent by Christ. Even as with time the Church has reserved the title “apostle” in a special way for the Twelve, Paul was such a pivotal figure in the spread of the gospel that the Church has also applied this title to him, calling him “the apostle to the Gentiles.”