The Saint who used to work as tax official

John of Damascus succeeded his father as one of the Muslim caliph’s tax officials. Later he became became a monk at Mar Saba, near Jerusalem, and there passed the rest of his life studying, writing, and preaching, acquiring the name “the Golden Orator” (Greek: Chrysorrhoas, "streaming with gold," (i.e., the golden speaker))

Practically all the information concerning the life of John of Damascus available to us today has been through the records of John, Patriarch of Jerusalem. Though these notes have served as the single source of biographical information, dating back to the tenth century, these writings have been noted by scholars as having an exuberant lack of detail from a historical point of view and a bloated writing style.

In the early 8th century, iconoclasm, a movement opposed to the veneration of icons (iconoclasts), gained acceptance in the Byzantine court. Patriarch of Constantinople issued his first edict against the veneration of images and their exhibition in public places. John of Damascus undertook a spirited defence of holy images. He not only attacked the Byzantine emperor, but adopted a simplified style that allowed the controversy to be followed by the common people.

Among his approximately 150 written works the most significant is Pēgē gnōseōs (“The Source of Knowledge”), a synthesis of Christian philosophy and doctrine that was influential in directing the course of medieval Latin thought and that became the principal textbook of Greek Orthodox theology. 

He is one of the first known Christian critics of Islam. John also claims to have read the Quran, or at least parts of it. He uses the plural "we", whether in reference to himself, or to a group of Christians that he belonged to who spoke to the Muslims, or in reference to Christians in general. 

In 1890, he was declared a Doctor of the Church by Pope Leo XIII.

References
https://www.britannica.com/biography/Saint-John-of-Damascus

https://www.catholic.org/saints/saint.php?saint_id=66

https://orthodoxwiki.org/John_of_Damascus