Today is the feast of St. John Damascene, Doctor of the Church.
Saint John Damascene (also known as St. John of Damscus) was born In Damascus, Syria around 676 AD into a rich family and spent the early years of his adult life serving as the official representative of the Christian community to the Muslim Caliph. He later abandoned this political task to join the monastery of St. Sabas near Jerusalem where he became a priest and ultimately bishop.
St. John Damascene is known as one of the last of the Fathers of the Church. He was a strong defender of the use of images (icons) in Christian worship against the iconoclasts and wrote a book "On the Orthodox Faith" that sums up the doctrinal heritage of the earlier Greek Fathers. In this great synthesis we find a systematic treatment of the central Christian doctrines, especially the Trinity, Creation, and the Incarnation. St. John Damascene's treatment of the Sacraments is also extensive, and his emphasis on the real bodily presence of Jesus in the Eucharist is very strong. Notable too in his teaching is a fully developed doctrine of the Blessed Virgin Mary including her perpetual virginity, her freedom from sin throughout the whole of her life, and her bodily assumption into heaven.
St. John Damascene's influence on later theology was considerable indeed. In the Latin Middle Ages, he was known to Peter Lombard and St. Thomas Aquinas. All throughout the Middle Ages his works were known and widely used by Eastern Christian Theologians, especially the Slavs. He died died some time between 754 and 787 AD and was declared a Doctor of the Church by Pope Leo XIII in 1890. His eloquent defense of images has deservedly procured him the title of "The Doctor of Christian Art."
To learn more about St. John Damascene, see: Catholic Culture, EWTN, and Catholic Encyclopedia.