Along the way to his martyrdom, he wrote seven letters to various Christian communities. Besides the New Testament, his letters are among the earliest Christian writings we have. His letters are therefore invaluable for learning about the state and development of early Christianity.
Most of us know a bit about first-century Christianity, since we're probably familiar with the writings of the New Testament. The Acts of the Apostles and Paul's letters, for example, show a fledgling church confronting its first challenges and controversies—all the while giving us a peak at early Christian beliefs and practices. But what happened after the Apostles left the scene?