Paul insisted on using the title apostle despite never having met Jesus and never being called personally by Jesus because of his vision on the road to Damascus. Paul claimed that the resurrected Jesus spoke to him and called him at that time. Taking the title "apostle" has clear political implications because the original apostles would have had the greatest authority within the growing Christian movement. They were, after all, the ones who had personally known and personally been called by Jesus. This would have put Paul in a secondary position at best, especially given his background as an opponent of Christianity.
Read Article: Saint Paul, the Thirteenth Apostle: Why Did Paul Call Himself an Apostle When He Never Met Jesus?