Byzantine historian, partisan of Arianism, a Christian heresy asserting the inferiority of Christ to God the Father. His church history, preserved in part, was the most extensive collection of Arian source texts assembled in a single work and furnished valuable data on the history, personalities, and intellectual milieu of theological controversy in the early church.

Philostorgius was the son of a staunch Arian and from the age of 20 studied in Constantinople and became a follower of Eunomius of Cyzicus, a leading exponent of extreme Arianism. This branch of the heresy stressed an absolute monotheism: only the Father is perfect God; the Son, Christ, is created.

Between 425 and 433 (during the reign of Emperor Theodosius II), Philostorgius wrote his church history in 12 books, after visiting Arian communities throughout the Eastern empire. The work, covering the period 300 to 425, was intended to continue the monumental Ecclesiastical History by the 4th-century chronicler Eusebius of Caesarea. In reality it constituted an apology for the radical Arian school. Beyond fragmentary references by Byzantine historians from the 9th to the 13th century, it has survived only in a summary and commentary in the Bibliotheca (“The Library,” or annotated bibliography) of Photius (Photius, Saint), the 9th-century scholarly patriarch of Constantinople. Although he acclaimed Philostorgius' style and diction, Photius charged him with obscurity and bias, particularly in his laudatory treatment of Eunomius and other Arian spokesmen and in his condemnation of orthodox theologians and emperors. Philostorgius refrained from attacking directly the celebrated orthodox leaders Gregory Nazianzene and Basil of Caesarea; he admitted the cogency of some of their refutations of heterodox Trinitarian theology but chided them for their criticism of his mentor, Eunomius. The History appealed to the cultured Greek because of its Arian emphasis on the rational intelligibility of Christian revelation. It also depicts the Arian response to the pagan accusation that Christianity influenced the political misfortunes of the Greco-Roman empire and civilization. Philostorgius countered that the lamentable collapse of classical culture into barbarism verified Christian apocalyptic teaching, or the predictions and signs portending the end of the world and the Second Coming of Christ.

Byzantine chronicles mention an apology for Christianity, written against the 3rd-century Neoplatonist Porphyry, but this tract has been lost. An English translation of The Ecclesiastical History of Philostorgius as Epitomized by Photius was done by E. Walford (1851). A critical edition of the Greek text was compiled by Joseph Bidez in the series Die Griechischen christlichen Schriftsteller der ersten drei Jahrhunderte, vol. 21 (1913; “The Greek Christian Writers”).