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John was raised in Damascus, and Arab Christian folklore holds that during his adolescence, John associated with the future Umayyad caliph Yazid I and the Taghlibi Christian court poet al-Akhtal.

One of the vitae describes his father's desire for him to "learn not only the books of the Muslims, but those of the Greeks as well." From this it has been suggested that John may have grown up bilingual. John does indeed show some knowledge of the Quran, which he criticizes harshly. (See Christianity and Islam.)

Other sources describe his education in Damascus as having been conducted in accordance with the principles of Hellenic education, termed "secular" by one source and "classical Christian" by another. One account identifies his tutor as a monk by the name of Cosmas, who had been kidnapped by Arabs from his home in Sicily, and for whom John's father paid a great price.

Under the instruction of Cosmas, who also taught John's orphan friend, Cosmas of Maiuma, John is said to have made great advances in music, astronomy and theology, soon rivalling Pythagoras in arithmetic and Euclid in geometry. As a refugee from Italy, Cosmas brought with him the scholarly traditions of Latin Christianity.

John possibly had a career as a civil servant for the Caliph in Damascus before his ordination.

He then became a priest and monk at the Mar Saba monastery near Jerusalem. One source suggests John left Damascus to become a monk around 706, when al-Walid I increased the Islamicisation of the Caliphate's administration. This is uncertain, as Muslim sources only mention that his father Sarjun (Sergius) left the administration around this time, and fail to name John at all. During the next two decades, culminating in the Siege of Constantinople (717-718), the Umayyad Caliphate progressively occupied the borderlands of the Byzantine Empire. An editor of John's works, Father Le Quien, has shown that John was already a monk at Mar Saba before the dispute over iconoclasm, explained below.

In the early 8th century, iconoclasm, a movement opposed to the veneration of icons, gained acceptance in the Byzantine court. In 726, despite the protests of Germanus, Patriarch of Constantinople, Emperor Leo III (who had forced his predecessor, Theodosius III, to abdicate and himself assumed the throne in 717 immediately before the great siege) issued his first edict against the veneration of images and their exhibition in public places.

All agree that John of Damascus undertook a spirited defence of holy images in three separate publications. The earliest of these works, his Apologetic Treatises against those Decrying the Holy Images, secured his reputation. He not only attacked the Byzantine emperor, but adopted a simplified style that allowed the controversy to be followed by the common people, stirring rebellion among the iconoclasts. Decades after his death, John's writings would play an important role during the Second Council of Nicaea (787), which convened to settle the icon dispute.

John's biography recounts at least one episode deemed improbable or legendary. Leo III reportedly sent forged documents to the caliph which implicated John in a plot to attack Damascus. The caliph then ordered John's right hand be cut off and hung up in public view. Some days afterwards, John asked for the restitution of his hand, and prayed fervently to the Theotokos before her icon: thereupon, his hand is said to have been miraculously restored. In gratitude for this miraculous healing, he attached a silver hand to the icon, which thereafter became known as the "Three-handed", or Tricheirousa. That icon is now located in the Hilandar monastery of the Holy Mountain.

Source: https://en.wikipedia.org/wiki/John_of_Damascus