"It has been said with truth," says the writer of the Article on Gregory of Nazianzus in the Dictionary of Christian Biography, "that these discourses would lose their chief charm in a translation....Critics have rivalled each other in the praises they have heaped upon them, but no praise is so high as that of the many Theologians who have found in them their own best thoughts. A Critic who cannot be accused of partiality towards Gregory has given in a few words perhaps the truest estimate of them: `A solidity of thought, the concentration of all that is spread through the writings of Hilary, Basil, and Athanasius, a flow of softened eloquence which does not halt or lose itself for a moment, an argument nervous without dryness on the one hand, and without useless ornament on the other, give these five Discourses a place to themselves among the monuments of this fine Genius, who was not always in the same degree free from grandiloquence and affectation. In a few pages, and in a few hours, Gregory has summed up and closed the controversy of a whole Century.'"  They were preached in the Church called Anastasia,  at Constantinople, between 379 and 381, and have gained for their author the title of The Theologian, which he shares with S. John the Evangelist alone. It should perhaps, however, be noted that the word is not here used in the wide and general sense in which we employ it, but in a narrower and more specific way, denoting emphatically the Defender of the Deity of the Logos. His principal opponents were the followers of Eunomius and Macedonius, and it is almost entirely against them that these Orations on Theology, or the Godhead of the Word and the Holy Ghost, are directed. The chief object of the Preacher in these and most other of his public utterances, is to maintain the Nicene Faith of the Trinity or Trinity of God; that is, the Doctrine that while there is but One Substance or Essence  in the Godhead, and by consequence God is in the most absolute sense One, yet God is not Unipersonal, but within this Undivided Unity there are three Self-determining Subjects or Persons, distinguished from one another by special characteristics (idiotetes) or personal properties—Father, Son, and Holy Ghost. With this object he entered into conflict with the heretics named above, who denied either the Consubstantiality of the Son with the Father, or the perfect Godhead and Personality of the Holy Ghost.
 De Broglie, "L'Eglise et l'Empire," v. 385.—"Ce sont autant de modeles dans l'art delicat d'imprimer la forme oratoire aux developpements philosophiques. Une pensee substantielle, formee de tous les sucs repandus dans les ecrits d'Hilaire, de Basile et d'Athanase; un courant d'eloquence temperee qui ne se ralentit, ni ne s'egare en aucun moment; une argumentation nerveuse sans secheresse, mais sans vaine parure d'ornements, font `a ces cinq discours une place `a part parmi les monuments de ce beau genie, auquel l'emphase et l'affectation ne furent pas toujours aussi etrangers. En quelques pages, et en quelques heures, Gregoire avait resume et clos la controverse de tout un siecle."
 See Prolegomena p. 171.
 "There is but one divine Essence or Substance; Father, Son, and Spirit, are one in essence, or consubstantial. They are in one another, inseparable, and cannot be conceived without each other. In this point the Nicene doctrine is thoroughly monotheistic, or monarchian, in distinction from tritheism, which is but a new form of the polytheism of the pagans. "The terms Essence (ousia) and Nature (phusis), in the philosophical sense, denote not an individual, a personality, but the Genus or Species; not Unum in Numero, but Ens Unum in Multis. All men are of the same substance, partake of the same human nature; though as persons and individuals they are very different. The term Homo-ousion, in its strict grammatical sense, differs from Mono-ousion or Touto-ousion, as well as from Hetero-ousion, and signifies not numerical identity, but equality of essence or community of nature among several beings. It is clearly thus used in the Chalcedonian Symbol, where it is said that Christ is `consubstantial (Homo-ousios) with the Father as touching the Godhead, and consubstantial with us (and yet individually distinct from us) as touching the Manhood.' But in the Divine Trinity consubstantiality denotes not only sameness of kind, but at the same time Numerical unity; not merely the Unum in Specie, but also the Unum in Numero. The three Persons are related to the Divine Substance not as three individuals to their species, as Abraham, Isaac, and Jacob, or Peter, John, and Paul, to human nature; they are only one God. The divine Substance is absolutely indivisible by reason of its simplicity, and absolutely inextensible and untransferable by reason of its infinity; whereas a corporeal substance can be divided, and the human nature can be multiplied by generation. Three Divine substances would limit and exclude each other, and therefore could not be infinite or absolute. The whole fulness of the one undivided Essence of God, with all its attributes, is in all the Persons of the Trinity, though in each in His own way; in the Father as Original Principle, in the Son by eternal Generation, in the Spirit by eternal Procession. The Church teaches not One Divine Essence and Three Persons, but One Essence In Three Persons. Father, Son, and Spirit cannot be conceived as Three separate individuals, but are in one another, and form a solidaric Unity." (Schaff, History of the Church, Nic. & Post-Nic. Period, Div. ii. p. 672.)
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Reference address : http://www.elpenor.org/gregory-nazianzen/introduction-theological-orations.asp