From Catholic Encyclopedia 1913:
Born at Damascus, about 676; died some time between 754 and 787. The only extant life of the saint is that by John, Patriarch of Jerusalem, which dates from the tenth century (P.G. XCIV, 429-90). This life is the single source from which have been drawn the materials of all his biographical notices. It is extremely unsatisfactory from the standpoint of historical criticism. An exasperating lack of detail, a pronounced legendary tendency, and a turgid style are its chief characteristics. Mansur was probably the name of John's father. What little is known of him indicates that he was a sterling Christian whose infidel environment made no impression on his religious fervour. Apparently his adhesion to Christian truth constituted no offence in the eyes of his Saracen countrymen, for he seems to have enjoyed their esteem in an eminent degree, and discharged the duties of chief financial officer for the caliph, Abdul Malek. The author of the life records the names of but two of his children, John and his half-brother Cosmas. When the future apologist had reached the age of twenty-three his father cast about for a Christian tutor capable of giving his sons the best education the age afforded. In this he was singularly fortunate. Standing one day in the market-place he discovered among the captives taken in a recent raid on the shores of Italy a Sicilian monk named Cosmas. Investigation proved him to be a man of deep and broad erudition. Through the influence of the caliph, Mansur secured the captive's liberty and appointed him tutor to his sons. Under the tutelage of Cosmas, John made such rapid progress that, in the enthusiastic language of his biographer, he soon equalled Diophantus in algebra and Euclid in geometry. Equal progress was made in music, astronomy, and theology.
On the death of his father, John Damascene was made protosymbulus, or chief councillor, of Damascus. It was during his incumbency of this office that the Church in the East began to be agitated by the first mutterings of the Iconoclast heresy. In 726, despite the protests of Germanus, Patriarch of Constantinople, Leo the Isaurian issued his first edict against the veneration of images. From his secure refuge in the caliph's court, John Damascene immediately entered the lists against him, in defence of this ancient usage of the Christians. Not only did he himself oppose the Byzantine monarch, but he also stirred the people to resistance. In 730 the Isaurian issued a second edict, in which he not only forbade the veneration of images, but even inhibited their exhibition in public places. To this royal decree the Damascene replied with even greater vigour than before, and by the adoption of a simpler style brought the Christian side of the controversy within the grasp of the common people. A third letter emphasized what he had already said and warned the emperor to beware of the consequences of this unlawful action. Naturally, these powerful apologies aroused the anger of the Byzantine emperor. Unable to reach the writer with physical force, he sought to encompass his destruction by strategy. Having secured an autograph letter written by John Damascene, he forged a letter, exactly similar in chirography, purporting to have been written by John to the Isaurian, and offering to betray into his hands the city of Damascus. The letter he sent to the caliph. Notwithstanding his councillor's earnest avowal of innocence, the latter accepted it as genuine and ordered that the hand that wrote it be severed at the wrist. The sentence was executed, but, according to his biographer, through the intervention of the Blessed Virgin, the amputated hand was miraculously restored.
The caliph, now convinced of John's innocence, would fain have reinstated him in his former office, but the Damascene had heard a call to a higher life, and with his foster-brother entered the monastery of St. Sabas, some eighteen miles south-east of Jerusalem. After the usual probation, John V, Patriarch of Jerusalem, conferred on him the office of the priesthood. In 754 the pseudo-Synod of Constantinople, convened at the command of Constantine Copronymus, the successor of Leo, confirmed the principles of the Iconoclasts and anathematized by name those who had conspicuously opposed them. But the largest measure of the council's spleen was reserved for John of Damascus. He was called a "cursed favourer of Saracens", a "traitorous worshipper of images", a "wronger of Jesus Christ", a "teacher of impiety", and a "bad interpreter of the Scriptures". At the emperor's command his name was written "Manzer" (Manzeros, a bastard). But the Seventh General Council of Nicea (787) made ample amends for the insults of his enemies, and Theophanes, writing in 813, tells us that he was surnamed Chrysorrhoas (golden stream) by his friends on account of his oratorical gifts. In the pontificate of Leo XIII he was enrolled among the doctors of the Church. His feast is celebrated on 27 March.
John of Damascus was the last of the Greek Fathers. His genius was not for original theological development, but for compilation of an encyclopedic character. In fact, the state of full development to which theological thought had been brought by the great Greek writers and councils left him little else than the work of an encyclopedist; and this work he performed in such manner as to merit the gratitude of all succeeding ages. Some consider him the precursor of the Scholastics, whilst others regard him as the first Scholastic, and his "De fide orthodoxa" as the first work of Scholasticism. The Arabians too, owe not a little of the fame of their philosophy to his inspiration. The most important and best known of all his works is that to which the author himself gave the name of "Fountain of Wisdom" (pege gnoseos). This work has always been held in the highest esteem in both the Catholic and Greek Churches. Its merit is not that of originality, for the author asserts, at the end of the second chapter of the "Dialectic", that it is not his purpose to set forth his own views, but rather to collate and epitomize in a single work the opinions of the great ecclesiastical writers who have gone before him. A special interest attaches to it for the reason that it is the first attempt at a summa theologica that has come down to us.
The "Fountain of Wisdom" is divided into three parts, namely, "Philosophical Chapters" (Kephalaia philosophika), "Concerning Heresy" (peri aipeseon), and "An Exact Exposition of the Orthodox Faith" (Ikdosis akribes tes orthodoxou pisteos). The title of the first book is somewhat too comprehensive for its contents and consequently is more commonly called "Dialectic". With the exception of the fifteen chapters that deal exclusively with logic, it has mostly to do with the ontology of Aristotle. It is largely a summary of the Categories of Aristotle with Porphyry's "Isagoge" (Eisagoge eis tas kategorias). It seems to have been John Damascene's purpose to give his readers only such philosophical knowledge as was necessary for understanding the subsequent parts of the "Fountain of Wisdom". For more than one reason the "Dialectic" is a work of unusual interest. In the first place, it is a record of the technical terminology used by the Greek Fathers, not only against the heretics, but also in the exposition of the Faith for the benefit of Christians. It is interesting, too, for the reason that it is a partial exposition of the "Organon", and the application of its methods to Catholic theology a century before the first Arabic translation of Aristotle made its appearance. The second part, "Concerning Heresy", is little more than a copy of a similar work by Epiphanius, brought up to date by John Damascene. The author indeed expressly disclaims originality except in the chapters devoted to Islamism, Iconoclasm, and Aposchitae. To the list of eighty heresies that constitute the "Panarion" of Epiphanius, he added twenty heresies that had sprung up since his time. In treating of Islamism he vigorously assails the immoral practices of Mohammed and the corrupt teachings inserted in the Koran to legalize the delinquencies of the prophet. Like Epiphanius, he brings the work to a close with a fervent profession of Faith. John's authorship of this book has been challenged, for the reason that the writer, in treating of Arianism, speaks of Arius, who died four centuries before the time of Damascene, as still living and working spiritual ruin among his people. The solution of the difficulty is to be found in the fact that John of Damascene did not epitomize the contents of the "Panarion", but copied it verbatim. Hence the passage referred to is in the exact words of Epiphanius himself, who was a contemporary of Arius....
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St. John Damascene, Doctor of the Church and priest4-Dec
From Catholic Encyclopedia 1913: