Basil the Great, also known as Saint Basil of Caesarea, was a 4th-century bishop and theologian who greatly influenced the development of Christian monasticism and the doctrine of the Trinity. Born around 330 in Caesarea (modern-day Turkey), he was a strong defender of Nicene Christianity against Arianism. Basil founded one of the earliest organized monasteries and wrote the Rule of St. Basil, a guide that still shapes Eastern Orthodox monastic life. He is venerated as a saint in both the Eastern and Western Christian traditions.