"Concerning the Orthodox Faith", the third book of the "Fountain of Wisdom", is the most important of John Damascene's writings and one of the most notable works of Christian antiquity. Its authority has always been great among the theologians of the East and West. Here, again, the author modestly disavows any claim of originality - any purpose to essay a new exposition of doctrinal truth. He assigns himself the less pretentious task of collecting in a single work the opinions of the ancient writers scattered through many volumes, and of systematizing and connecting them in a logical whole. It is no small credit to John of Damascus that he was able to give to the Church in the eighth century its first summary of connected theological opinions. At the command of Eugenius III it was rendered into Latin by Burgundio of Pisa, in 1150, shortly before Peter Lombard's "Book of Sentences" appeared. This translation was used by Peter Lombard and St. Thomas Aquinas, as well as by other theologians, till the Humanists rejected it for a more elegant one. The author follows the same order as does Theodoret of Cyrus in his "Epitome of Christian Doctrine". But, while he imitates the general plan of Theodoret, he does not make use of his method. He quotes, not only form the pages of Holy Writ, but also from the writings of the Fathers. As a result, his work is an inexhaustible thesaurus of tradition which became the standard for the great Scholastics who followed. In particular, he draws generously from Gregory of Nazianzus, whose works he seems to have absorbed, from Basil, Gregory of Nyssa, Cyril of Alexandria, Leo the Great, Athanasius, John Chrysostum, and Epiphanius. The work is divided into four books.