An Apostle is one of the original 12 disciples called by Jesus to
preach the gospel: Simon Peter, the brothers James and John,
Andrew, Philip, Bartholomew, Matthew, Thomas, James the son of
Alpheus, Thaddaeus, Simon the Zealot, Judas Iscariot.
As Paul never met Jesus, many Galatians and Corinthian Christians
actively challanged Pauls right to even teach, and his right to
call himself an Apostle.