St. Gregory of Nazianzus, also known as Gregory the Theologian, was a prominent early Christian bishop and theologian born around 329 AD in Cappadocia, modern-day Turkey. He played a significant role in the development of Christian doctrine, particularly concerning the nature of the Trinity. Gregory was a key figure at the First Council of Constantinople in 381 AD, where he defended the divinity of the Holy Spirit. Gregory was also known for his eloquent sermons and writings, which contributed to the establishment of Nicene Christianity. He served as the Archbishop of Constantinople and is recognized as one of the Cappadocian Fathers, alongside Basil the Great and Gregory of Nyssa. His feast day is celebrated on January 2 in the Western Church and on January 30 in the Eastern Orthodox Church.