The Gnostic Origins of “Homoousios”
Hundreds, possibly thousands, of academic works have been written on the subject of Gnosticism, an esoteric movement that was a threat to the early church. It suffices for our purposes to give two brief explanations of Gnosticism from two references:
Oxford Dictionary of English (2010): “Gnosticism, a prominent heretical movement of the 2nd-century Christian Church, partly of pre-Christian origin. Gnostic doctrine taught that the world was created and ruled by a lesser divinity, the demiurge, and that Christ was an emissary of the remote supreme divine being, esoteric knowledge (gnosis) of whom enabled the redemption of the human spirit.”
Encarta 2007 Encyclopedia: “Gnosticism, esoteric religious movement that flourished during the 2nd and 3rd centuries AD and presented a major challenge to orthodox Christianity. Most Gnostic sects professed Christianity, but their beliefs sharply diverged from those of the majority of Christians in the early church (see Heresy). The term gnosticism is derived from the Greek word gnosis (‘revealed knowledge’). To its adherents, Gnosticism promised a secret knowledge of the divine realm. Sparks or seeds of the Divine Being fell from this transcendent realm into the material universe, which is wholly evil, and were imprisoned in human bodies. Reawakened by knowledge, the divine element in humanity can return to its proper home in the transcendent spiritual realm.” (“Gnosticism,” paragraph 1, Encarta 2007)
Various sources, ancient and modern, have touched on the Gnostic origins of the word homoousios (ὁμοούσιος, one in substance) that was controversially adopted by the Council of Nicaea to assert that the Father and the Son are of “one substance” or the “same essence”. Its Gnostic origins was one of the reasons that made the word suspect and the target of criticism, even by some who later acceded to the Nicene creed, in the debates leading up to the Nicene formulation.
A masterly and meticulously documented discussion of the Gnostic origins of homoousios is found in a paragraph of the Wikipedia article Homoousian under the heading “Pre-Nicene use of the term”: 
Pre-Nicene use of the term
The term ὁμοούσιος (homoousios) had been used before its adoption by the Nicene theology. The Gnostics were the first theologians to use the word homoousios, while before the Gnostics there is no trace at all of its existence. The early church theologians were probably made aware of this concept, and thus of the doctrine of emanation, by the Gnostics. In Gnostic texts the word homoousios is used with these meanings: (1) identity of substance between generating and generated; (2) identity of substance between things generated of the same substance; (3) identity of substance between the partners of a syzygy. For example, Basilides, the first known Gnostic thinker to use homoousios in the first half of the 2nd century, speaks of a threefold sonship consubstantial with the god who is not.  The Valentinian Gnostic Ptolemy claims in his letter to Flora that it is the nature of the good God to beget and bring forth only beings similar to, and consubstantial with himself.  Homoousios was already in current use by the 2nd-century Gnostics, and through their works it became known to the orthodox heresiologists, though this Gnostic use of the term had no reference to the specific relationship between Father and Son, as is the case in the Nicene Creed.
 The Search for the Christian Doctrine of God: The Arian Controversy 318-381, R.P.C. Hanson, chapter 7, pp.190-202.
 We quote the second paragraph of Wikipedia article “Homoousian” as it was on February 20, 2013 at http://en.wikipedia.org/wiki/Homoousian. The four footnotes in this excerpt are here included in their entirety and without alteration except for a change in footnote numbers, originally 1 to 4, but changed to higher footnote numbers to conform to the footnote numbering sequence of the present book.
 Adolf von Harnack, Dogmengeschichte, 1:284-85, n.3; 2:232-34, n.4. Ignacio Ortiz de Urbina, “L’homoousios preniceno,” Orientalia Christiana Periodica 8 (1942): 194-209; Ignacio Ortiz de Urbina, El Simbolo Niceno (Madrid: Consejo Superior de Investigaciones Cientificas, 1947), 183-202. Luis M. Mendizabal, “El Homoousios Preniceno Extraeclesiastico,” Esthdios Eclesiasticos 30 (1956): 147-96. George Leonard Prestige, God in Patristic Thought (London: SPCK, 1936; 2d ed., 1952), 197-218. Peter Gerlitz, Aufierchristliche Einflilsse auf die Entwicklung des christlichen. Trinitatsdogmas, zugleich ein religions- und dogmengeschichtlicher Versuch zur Erklarung der Herkunft der Homousie (Leiden: Brill, 1963), 193-221. Ephrem Boularand, L’heresie d’Arius et la “foi” de Nicke, vol. 2, “La “foi” de Nicee” (Paris: Letouzey & Ane, 1972), 331-53. J.N.D. Kelly, Early Christian Creeds, 3d ed. (London: Longman, 1972), 245. Frauke Dinsen, Homoousios. Die Geschichte des Begriffs bis zum Konzil von Konstantinopel (381), Diss. Kiel 1976, 4-11. Christopher Stead, Divine Substance, 190-202.
 Aloys Grillmeier, Christ in Christian Tradition, vol. 1, From the Apostolic Age to Chalcedon (451) (London: Mowbrays, 1975), p.109.
 According to Hippolytus: “Υἱότης τριμερής, κατὰ πάντα τῷ οὐκ ὄντι θεῷ ὁμοούσιος”. (Refutatio omnium haeresium 7:22) See also, for the Gnostic use of the term, Miroslav Marcovich in Patristische Texte und Studien, 25 (Berlin: W. de Gruyter, 1986), 290f. V,8,10 (156); V,17,6.10 (186 f.).
 According to Epiphanius: “Τοῦ ἀγαθοῦ φύσιν ἔχοντος τὰ ὅμοια ἑαυτῷ καὶ ὁμοούσια γεννᾶν τε καὶ προφέρειν”. (Panarion 33:7,8)
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