Final answer:
The letters of Paul were presumably written by St. Paul, a key figure in early Christianity known for his missionary work and theological interpretations which shaped the development of the Christian canon and beliefs.
Step-by-step explanation:
The letters of Paul, which are earlier texts than the four gospels and The Acts of the Apostles, were presumably written by St. Paul, also known as Saul prior to his conversion to Christianity. As a prominent figure in the spread of Christianity, St. Paul was a Jewish leader who underwent a life-changing conversion experience; he then became a key Christian evangelist. Paul's epistles are central in conveying his interpretation of Jesus Christ's message and the significance of Jesus's death, which he believed was a divine plan to absolve human sin.
Early Christianity, due to the diversity of belief and interpretation of Jesus's teachings, had no single canon of scriptures immediately after the death of Jesus. Over time, Paul's letters and other texts became instrumental in forming the Christian canon, shaping the faith as it spread throughout the Greco-Roman world.