Butler has this to say about Rupert:
HE was, by birth, a Frenchman, and of royal blood; but still more illustrious for his learning, and the extraordinary virtues he practised from his youth. He exercised himself in austere fasting, watching, and other mortifications; was a great lover of chastity and temperance; and so charitable as always to impoverish himself to enrich the poor. His reputation drew persons from remote provinces to receive his advice and instructions. He removed all their doubts and scruples, comforted the afflicted, cured the sick, and healed the disorders of souls. So distinguished a merit raised him to the episcopal see of Worms. But that people, being for the most part, idolaters, could not bear the lustre of such sanctity, which condemned their irregularities and superstitions. They beat him with rods, loaded him with all manner of outrages, and expelled him the city. But God prepared for him another harvest. Theodon, duke of Bavaria, hearing of his reputation and miracles, sent messengers to him, earnestly beseeching him to come and preach the gospel to the Baioarians, or Bavarians. This happened two years after his expulsion from Worms; during which interval he had made a journey to Rome. He was received at Ratisbon by Theodon and his court with all possible distinction, in 697, and found the hearts both of the nobles and people docile to the Word of God. The Christian faith had been planted in that country two hundred years before, by St. Severinus, the apostle of Noricum. After his death, heresies and heathenish superstitions had entirely extinguished the light of the gospel. Bagintrude, sister of duke Theodon, being a Christian, disposed her brother and the whole country to receive the faith. Rupert, with the help of other zealous priests, whom he had brought with him, instructed, and, after a general fast, baptized the duke Theodon and the lords and people of the whole country. God confirmed his preaching by many miracles. He converted also to Christianity the neighbouring nations. After Ratisbon, the capital, the second chief seat of his labours was Laureacum, now called Lorch, 1 where he healed several diseases by prayer, and made many converts. However, it was not Lorch, nor the old Reginum, thence called Regensbourg, now Ratisbon, the capital of all those provinces, that was pitched upon to be the seat of the saint’s bishopric, but the old Juvavia, then almost in ruins, since rebuilt and called Saltzbourg. The duke Theodon adorned and enriched it with many magnificent donations, which enabled St. Rupert to found there several rich churches and monasteries. After that prince’s death, his son, Theodebert, or Diotper, inheriting his zeal and piety, augmented considerably the revenues of this church. St. Rupert took a journey into France to procure a new supply of able labourers, and brought back to Saltzbourg twelve holy missionaries, with his niece St. Erentrude, a virgin consecrated to God, for whom he built a great monastery, called Nunberg, of which she was the first abbess. 2 St. Rupert laboured several years in this see, and died happily on Easter-day, which fell that year on the 27th of March, after he had said mass and preached; on which day the Roman and other Martyrologies mention him. His principal festival is kept with the greatest solemnity in Austria and Bavaria on the 25th of September, the day of one of the translations of his relics, which are kept in the church under his name in Saltzbourg. Mabillon and Bulteau, upon no slight grounds, think this saint to have lived a whole century later than is commonly supposed, and that he founded the church of Saltzbourg about the year 700. See his life, published by Canisius, Henschenius, and Mabillon, with the notes of the last-mentioned editor.
Orthodox Wiki has this to say about John of Damascus:
Although he was brought up under the Muslim rule of Damascus, this was not to affect his or his family's Christian faith or cause any grievances with the Muslim countrymen who held him in high esteem. To the extent that his father held a high hereditary public office with duties of chief financial officer for the caliph, Abdul Malekunder, apparently as head of the tax department for Syria.
When John reached the age of twenty-three, his father sought out to find a Christian tutor who could provide the best education for his children available at the time. Records show that while spending some time in the market place John's father came across several captives, imprisoned as a result of a raid for prisoners of war that had taken place in the coasts of Italy. This man, a Sicilian monk by the name of Cosmas, turned out to be an erudite of great knowledge and wisdom. John's father arranged for the release of this man and appointed him tutor to his son. Under the instruction of Cosmas, John made great advances in fields of study such as music, astronomy and theology. According to his biographer, he soon equaled Diophantus in algebra and Euclid in geometry.
In spite of his Christian background, his family held a high hereditary public office with the Moslem rulers of Damascus, led by caliph Abd al-Malik. He succeeded his father in his position upon his death; John de Damascene was made protosymbullus, or chief councilor of Damascus.
It was around his term in office that burst of insurgence by the iconoclasts began to appear in the form of heresy, actions which disturbed the Church of the East. In 726, in disregard of the protests of Germanus, Patriarch of Constantinople, Emperor Leo the Isaurian issued his first edict against the veneration of images and their exhibition in public places. A talented writer and in the secure surroundings of the caliph's court, John de Damascene initiated his literary defense against the monarch in three Apologetic Treatises against those Decrying the Holy Images. This was the earliest of his works and the one which earned him a reputation. Not only did he attack the monarch, but his use of a simpler witting style brought the controversy to the common people, inciting revolt among those of Christian faith.
Unable to punish the writer openly, Leo the Isaurian managed to get possession of a manuscript written and signed by John de Damascene, which he used to forge a letter from John to the Isaurian monarch offering to betray into his hands the city of Damascus. Despite John's earnest advocation to his innocence, the caliph dismissed his plea and discharged him from his post, ordering his right hand, which he used for writing, to be severed at the wrist.
According to the 10th-century biography, his hand was miraculously restored after fervent prayer before an icon of the Virgin Mary. At this point the caliph is said to have been convinced of his innocence and inclined to reinstate him to his former office. However, John then retired to the Monastery of Saint Sabbas near Jerusalem, where he continued to produce a stream of commentaries, hymns and apologetic writings, including the Oktoechos (the Church's service book of eight tones) and An Exact Exposition of the Orthodox Faith, a summary of the dogmatic writings of the Early Church Fathers.