Who was the Apostle Paul? In fact, his name wasn't originally Paul, but SAUL. And he wasn't even a Christian. He was originally a leading Pharisee of great classical and rabbinical education, and a strict follower of the Law. Paul had hated Christianity and its followers, and was dedicated to killing and wiping out what he had long viewed as a threatening, corrupt and evil cult. Eventually, his anti-Christian zealousness had brought him considerable prestige as an up-and-coming "rockstar" defending the traditional teachings and practices of the Jewish Law. But after encountering the risen Jesus on the road to Damascus, Paul was temporarily blinded and confronted by Jesus (See Acts 9). So, what changed Paul from being a person with a wide reputation for severely persecuting the early Christians, to abandoning all of his stature and prestige as an important Pharisee, to actually EMBRACING Jesus as his Lord and becoming an integral instrument God used to build, teach and organize much of His first-century Church? And not only did Paul lose his growing stature amongst the religious establishment, he also put his life in great danger by abandoning the Pharisees for a life of following Jesus. Eventually, of the 27 Greek-language New Testament books, Paul wrote 13 of them - in the form of letters that eventually were copied and spread across the early churches. Paul wrote the books of Romans, 1 and 2 Corinthians, Galatians, Ephesians, Philippians, Colossians, 1 and 2 Thessalonians, 1 and 2 Timothy, Titus, and Philemon.
See this excellent overview of the Apostle Paul's life, in this 20-minute video:
https://www.youtube.com/watch?v=9_WG7pR9Z8I