John spent most of his life in the monastery of St. Sabas, near Jerusalem, and all of his life under Muslim rule, indeed, protected by it. He was born in Damascus, received a classical and theological education, and followed his father in a government position under the Arabs. After a few years he resigned and went to the monastery of St. Sabas.
He is famous in three areas. First, he is known for his writings against the iconoclasts, who opposed the veneration of images. Paradoxically, it was the Eastern Christian emperor Leo who forbade the practice, and it was because John lived in Muslim territory that his enemies could not silence him. Second, he is famous for his treatise, Exposition of the Orthodox Faith, a summary of the Greek Fathers (of which he became the last). It is said that this book is for Eastern schools what the Summa of Aquinas became for the West. Thirdly, he is known as a poet, one of the two greatest of the Eastern Church, the other being Romanus the Melodist. His devotion to the Blessed Mother and his sermons on her feasts are well known.