Γρηγόριος ο Θεολόγος, also known as Gregory of Nazianzus, was a prominent Christian theologian and one of the Cappadocian Fathers. Born around 329 AD in Nazianzus, he played a crucial role in the development of early Christian doctrine, particularly concerning the nature of the Trinity. His eloquent sermons and writings helped shape the understanding of the relationship between God the Father, God the Son, and God the Holy Spirit. He served as the Archbishop of Constantinople and was influential in the fight against Arianism, a belief that denied the divinity of Christ. Gregory's contributions to the First Council of Constantinople in 381 AD were significant in affirming the Nicene Creed. He is celebrated as a Doctor of the Church and is commemorated on January 25 in the Eastern Orthodox Church.