He lived near the end of patristic development of church dogma, and is considered the last of the Eastern Orthodox Doctors of the Church, being referred sometimes specifically as the Doctor of the Assumption because of his writing on the Assumption of Mary.
He spoke out in contrast to the Eastern tradition of iconoclasm. He wrote three (that we know of) works defending icons:
You see that He forbids image-making on account of idolatry, and that it is impossible to make an image of the immeasurable, uncircumscribed, invisible God. You have not seen the likeness of Him, the Scripture says, and this was St Paul’s testimony as he stood in the midst of the Areopagus: ‘Being, therefore, the offspring of God, we must not suppose the divinity to be like unto gold, or silver, or stone, the graving of art, and device of man.’
These injunctions were given to the Jews on account of their proneness to idolatry. Now we, on the contrary, are no longer in leading strings. Speaking theologically, it is given to us to avoid superstitious error, to be with God in the knowledge of the truth, to worship God alone, to enjoy the fulness of His knowledge. We have passed the stage of infancy, and reached the perfection of manhood. We receive our habit of mind from God, and know what may be imaged and what may not. [link]
The anti-Semitism is not unique. Other works of his show strong hostility to other groups: Against the Jacobites; Against the Nestorians; Dialogue against the Manichees; On the Faith, Against the Nestorians; On the Two Wills of Christ (Against the Monothelites); as well as the straightforward On Right Thinking.
He was also, unsurprisingly, opposed to Islam; one of the first known Christian writers to attack it. In Concerning Heresy he claims Muslims first worshipped Aphrodite, and that Mohammad learned Christianity from an Arian monk instead of true Christianity. Also, he criticizes the claim that Mohammad received the Koran from God in his sleep, because there were no witnesses. Moses received the Torah in front of the Israelites, Jesus was foretold by the Old Testament, but no witnesses exist to support Mohammad's claims, and no prophecies in the Bible foretold Mohammad.
John was also a promoter of perichoresis, the idea that the members of the Trinity are constantly "going around" each other, endlessly interacting and being intertwined. This sounds obvious (maybe) to anyone raised in a Christian environment, but pre-Nicene Councils, focus on the Trinity was often on distinguishing between the three to explain why three were needed. Perichoresis ties their being/existence closer together.
John of Damascus died 4 December 749. He is considered a saint in the Catholic Church, as well as Eastern and Ethiopian Orthodox Churches, the Anglican Communion, and Lutheranism. His feast day is 4 December and 27 March. He is there patron of pharmacists, theology students, and icon painters!
As mentioned above, John's writings helped define the dogma of the early Church. Next I want to go a little deeper into his unofficial title "Doctor of the Assumption."
