BardaiṢĀn, an early teacher of Christianity in Mesopotamia, the writer of numerous Syriac works which have entirely perished (with one possible exception, the Hymn of the Soul in the Acts of Thomas), and the founder of a school which was soon branded as heretical. According to the trustworthy Chronicle of Edessa, he was born in that city on the 11th Tammuz (July), A.D. 154. His parents were of rank and probably pagan; according to Barhebraeus, he was in youth a priest in a heathen temple at Mabbōg. Another probable tradition asserts that he shared the education of a royal prince who afterwards became king of Edessa - perhaps Abgar bar Manu, who reigned 202-217. He is said to have converted the prince to Christianity, and may have had an important share in christianizing the city. Epiphanius and Barhebraeus assert that he was first an orthodox Christian and afterwards an adherent of Valentinus; but Eusebius and the Armenian Moses of Chorene reverse the order, stating that in his later days he largely, but not completely, purged himself of his earlier errors.
The earliest works attributed to him (by Eusebius and others) are polemical dialogues against Marcionism and other heresies; these were afterwards translated into Greek. He also wrote, probably under Caracalla, an apology for the Christian religion in a time of persecution. But his greatest title to fame was furnished by his hymns, which, according to St Ephrem, numbered 150 and were composed in imitation of the Davidic psalter. He thus became the father of Syriac hymnology, and from the favour enjoyed by his poems during the century and a half that intervened between him and St Ephrem we may conclude that he possessed original poetic genius. This would be clearly proved if (as is not unlikely) the beautiful Hymn of the Soul incorporated in the apocryphal Acts of Thomas could be regarded as proceeding from his pen; it is practically the only piece of real poetry in Syriac that has come down to us. Perhaps owing to the persecution under Caracalla mentioned above, Bardaiṣān for a time retreated into Armenia, and is said to have there preached Christianity with indifferent success, and also to have composed a history of the Armenian kings.
Porphyry states that on one occasion at Edessa he interviewed an Indian deputation who had been sent to the Roman emperor, and questioned them as to the nature of Indian religion. He was undoubtedly a man of wide culture. He died (according to the patriarch Michael) in 222.
For our knowledge of Bardaiṣān's doctrine we are mainly dependent on the hostile witness of St Ephrem, and on statements by Greek writers who had no acquaintance with his works in their original form. His teaching had certain affinities with gnosticism. Thus he certainly denied the resurrection of the body; and so far as we can judge by the obscure quotations from his hymns furnished by St Ephrem he explained the origin of the world by a process of emanation from the supreme God whom he called "the Father of the living." On the other hand the dialogue known as the Book of the Laws of the Countries, which was written by a disciple and is quoted by Eusebius as a genuine exposition of the master's teaching - while it recognizes the influence of the celestial bodies over the body of man and throughout the material sphere and attributes to them a certain delegated authority - upholds the freedom of the human will and can in the main be reconciled with orthodox Christian teaching. On this M. Nau has based his effort (see Une Biographie inédite de Bardesane l'astrologue, Paris, 1897; Le Livre des lois des pays, Paris, 1899) to clear Bardaiṣān of the reproach of gnosticism, maintaining that the charge of heresy arises from a misunderstanding of certain astrological speculations.
It must be admitted that it is impossible to reconstruct Bardaiṣān's system from the few fragments remaining of his own work and therefore a certain verdict cannot be given. But the ancient testimony to the connexion of Bardaiṣān with Valentinianism is strong, and the dialogue probably represents a modification of Bardesanist teaching in the direction of orthodoxy. The later adherents of the school appear to have moved towards a Manichean dualism.
The subject is exhaustively discussed in Hort's article "Bardaisan" in Dict. Christ. Biog., and a full collection of the ancient testimonies will be found in Harnack's Altchristliche Litteratur, vol. i. pp. 184 ff.
 The Book of the Laws of the Countries, referred to below, is the work of a disciple of Bardaiṣān.
 Even Ephrem allows that Bardaiṣān was in principle a monotheist.