Plato considered heterodoxy as error, not as a simple departure from orthodoxy. Yet the ancient Church did not particularly attach itself to these denomina- tions of orthodoxy and heterodoxy as designating the contrast between the Christian truth and its opposite, for its doctrines were not yet firmly enough established. But as they gradually came to be more strictly defined, that which agreed with the decisions of the Church was called orthodox, and whatever differed from them heterodox. The notion of orthodoxy commenced only to acquire real power when the Church attained a se- cure footing in the state. We find the expression often used by Eusebius, Athanasius (whom Epiphanius calls the father of orthodoxy, I/ai: Ixix, c. 2), etc., and also among the Latins, e. g. in the writings of Jerome. Isi- dore of Hispalis says in the Ongines (vii, 14) : " Orthodox- us est recte credens et ut credit recte vivens." The Church as the embodiment of religion in the com- munity needs a firmly established doctrine as its basis; it no longer leaves the individual free to believe as he chooses. Unity of doctrine with the Church, or at least the acceptance of its fundamental principles, con- stitutes orthodoxy, departure from them is heterodoxy. A tendency to the use of tliese words was already ap- parent in the ancient Church, for we find Ignatius in the beginning of the 2d century designates those who depart from the general faith, as tauglit and supported by the bishops, as iripoSoKovvriQ (A d Smyrn. c. 6), and warns his readers against being led into error rale trt- poSo^iaic (Ad Magn. c. 8). liut he uses them more in the etymological than in the ecclesiastical sense. The ecclesiastical use of them did not become general before the 4th centtirj-, when the regula verilatis gradually acquired a more objective form in the canon of Scrip- ture, in the confessions of the Church, the decrees of the
synods, and the assertion of the Church possessing the standard of truth. In cases of uncertainty, the Church or the synods decided as to what was conformed to the doctrine of the Church (orthodox), and what contrary to it (heterodox). Thus it gradually proclaimed more and more loudly, especially in the East, that the doc- trine it taught constituted orthodoxj', and that every doctrine differing from it was heterodoxy.
This question of orthodoxy twice attained paramount importance in the Church. First in the difficulties con- cerning the dogma and ecclesiastical usages which, more from an outward impulse than from inner reasons, led to a separation between the Eastern and the West- ern churches. In these discussions, and particularly on that concerning images, the Greek Chut'ch always based itself on its antiquity and its orthodoxy, till in the course of the dispute the toprt) -ijg op^olo^iaq was es- tablished in 842, which led to the Greek Church as- suming specifically the name of orthodox, which it still maintains. The first formal exposition of its dogmas by John of Damascus (732) had already borne the title of tK^tmg rTjg opSroSo^ov Triarsiog, which was also the case with other distinguished dogmatic works after- wards, such as Euthymius Zigadenus's TravowXia coyf.ia- TiKi) rrjg op^oSu^ov Triareiog, and Nicetas Acominatus's ^tjaavpog bp^oSoS.iag. The Greek Church consequent- ly claims to possess the absolute truth, which she pre- serves without attempting to develop it, like a miser his treasure, while she considers all other Christian churches as heterodox, schismatic, and heretical. This is evinced in all official acts and documents of the Greek Church, as also from the generally received confession of the archbishop of Kief, Peter Mogilas, which bears the inscription 'OpSr6Sot,og bfioXoyia Tijg KaBoXiKiig Kai cnroaToXiKr]g iKKXr)(!'iag dva-oXiKrig. See '^chtookh, Kirchengesch. xvii, 466 sq. ; Marheineke, Ueb. d. Ursprung u. d. Entwickelung d. Oi'thodoxie u. Ueterodoxie, etc., in Daub u. Creutzer, Studien, iii, 1807.
The second occasion when the question of orthodoxy acquired such importance was at the time of Luther's Reformation. The whole body of doctrine was revised and determined down to the most minute dogmatic definitions. The adherents of the Reformation in the 16th century were from the first obliged to defend themselves against the accusation of heresy and neol- ogism. They were thus obliged to prove their con- formity with the ancient Church, and therefore their orthodoxy. But as on this point there was no ecclesi- astical authority to refer to, every member of the Prot- estant Church was obliged all the more diligently to prove his unity of doctrine with the true Church of Christ by the only valid standard, Scripture, and to re- ject from his association those who did not conform to that standard. The disputes which preceded the draw- ing up of the Formula of Concord greatly strengthened this feeling, and soon those alone were considered or- thodox who accepted every article of that formula. The zeal of the contest magnified the importance of the mooted points until it led almost to a separation. The orthodox party considered that the possession of the absolute truth was sufficient, without absolute puri- ty of life; it was a time of dead orthodoxy. There were certainly men of active and living piety in the party, but the paramount consideration was that of conformity to the doctrine of the Church, so that thoroughly worldly men who accepted fully every article of the formula were in high honor in tlie Church; while such men as John Arnd, Spencr, (iott- fried Arnold, could not atone by their piety for their want of conformity on some points, and were violently attacked by distinguished orthodox teachers. All het- erodoxy was then considered as heresy, i. e. regarded as attacking the very foundation of religious truth. This tendency was strenuously opposed by the gentle and learned G. Calixtus, and the pious and active Spener. Pietism, which arose about that time, aided in the work — although opposed also by the followers