John Damascene was one of the last great church fathers to live before the great schism which separated Christianity between its Orthodox and Catholic branches. In both churches Damascene is revered as a great saint and theologian, and is a doctor of the Catholic Church.
Damascene wrote several works, including anti-heresy treatises, philosophical works, and the classical exposition of Christan theology called 'On Orthodox Theology', which outlines the Christian understanding of God. Damascene also defended the use of icons in Churches, against iconoclasts who desired to implement the Old Testament commandment about the destruction of images of the divine. The centrality of icons to the theology and faith of the Orthodox Church today is due in large part to Damascene's famous defence.
Like most Easterns, Damascene emphasizes the incomprehensibility of God's nature (the only thing we can know about God is his infinity and incomprehensibility) and his faith is deeply mystical at times. However, he remains essential reading for the student of church history and theology.
Louth (who has also written works on Dionysius and Christian mysticism) writes a readable introduction to this great Father's thought.