Paul was an apostle, with Apostolic authority. He knew the Lord in a personal way. He was a planter of churches and an authority on correct doctrine. His friend Apollos probably was the author of the letter to the Hebrews, with Pauline consultation. Paul was known to all of the disciples as well as people like Clement of Rome. The letters or Epistles attributed to Paul comprise a substantial portion of the New Testament. He was martyred in Rome.
Augustine of Hippo was a 4th century theologian and an important one. His writing was excellent and his ideas philosophically interesting. His personal life and experiences and conversion to Christianity comprise much of his works. Theologically his most important position may have been the opinion that mankind’s nature with original sin is totally depraved, while another theologian named Pelagius had the point of view that it is only partially so.
Augustine interpreted scripture centuries after the crucifixion of Christ. Paul lived in the first century and was the Apostle to the Gentiles. His letters are scripture. Augustine was a philosopher of a kind; a neo-Platonist before conversion to Christianity. Paul was a Jewish Pharisee and student of Gamaliel before Christ appeared to him on the road to Damascus.
The way theological positions are hermeneutically interpreted from scripture determines church doctrine and informs church dogmatics. Augustine’s ideas are regarded as more accurate than those of Pelagius.
The Catholic church is still concerned about Pelagianism- quote from the Wikipedia- ” in a letter of February 2018 titled Placuit Deo, stating, “A new form of Pelagianism is spreading in our days, one in which the individual, understood to be radically autonomous, presumes to save oneself, without recognizing that, at the deepest level of being, he or she derives from God and from others.”