In the Fifth he deals with the doctrine of the Holy Ghost.  The heresy of Arius was at first directly concerned only with the Person of our Lord, though not without a side-glance at that also of the Holy Ghost.  The Council of Nicæa had confined itself to the first question, and its Creed ended with, “We believe in the Holy Ghost.”  This, it was afterwards argued, was enough to proclaim His Divinity, and so Gregory argues in this Oration, “If He be only a creature, how do we believe on Him, how are we made perfect in Him, for the first of these belongs to Deity, the second may be said of anything” (c. vi.).  The reason, however, that the Great Synod made no express definition on the point seems to have been that the controversy had not yet been carried so far in direct terms (cf. S. Basil, Epp. lxxviii. ccclxxxvii.).  But fifty years later the growth of the heresy rendered a definition of the Church’s faith on this point needful; and in 363, on his return from his fourth period of exile, S. Athanasius held a provincial Synod at Alexandria, in whose Synodical Letter to the Emperor Jovian the Godhead of the Holy Ghost is maintained in terms which, as Canon Bright says, partly anticipate the language of the Creed of Constantinople (Dict. Biog. Art. Athanasius).  The new development of the heresy had begun to appear at Constantinople as well as in Thrace and Asia Minor.  Macedonius, a Semi-Arian, had been elected Bishop of Constantinople in 341, and in spite of violent opposition, which he met by still more violent measures, had maintained his position till 360, when he was deposed and driven out by the Anomœan Arians.  He then in his retirement became the leader of the Semi-Arian party.  Accepting the statement that the Son was Like in Essence to the Father, he would not concede even this to the Holy Ghost, but declared Him to be a mere creature (Thdt. Hist. Eccl. ii. 6), and the servant or minister of the Son; applying to Him terms which without error could only be used of the Angels (Sozomen. H. E. iv. 27).  His followers were known as Macedonians, or sometimes Marathonians, from a certain Marathonius, formerly a Paymaster of the Prætorian Guards, who had become a Deacon of Constantinople, and, having done much in the way of founding and maintaining Monastic Houses and Houses of Charity in the City, was consecrated by Macedonius as Bishop of Nicomedia.  They were also known as Pneumatomachi, from the nature of their Heresy.  A controversy had now begun to arise as to the precise position which the true faith was to assign to the Holy Spirit.  There were those who left it doubtful whether He had indeed a separate Personality, or whether He were not rather a mere Influence or Activity of the Father and the Son.  Gregory tells us how, when he came to the Metropolis, he found the wildest confusion prevalent.  Some, he says, conceived of the Holy Ghost as a mere Energy of God, others thought Him a Creature, others believed Him to be God; while many out of an alleged reverence for Holy Scripture, hesitated to give Him the Name of God.  To this last class belonged, according to Socrates (H. E. ii. 45), Eustathius, who had been ejected from the Bishopric of Sebasteia in Pontus.  He refused to admit that the Holy Spirit is God, while yet He did not dare to affirm that He is a mere creature.  When Gregory proceeded to preach the Deity of the Spirit, he was accused of introducing a strange and unscriptural god, because, as he acknowledges, the letter of the Bible is not so clear on the doctrine of the Spirit as it is on that of the Son.  But he points out that it is possible to be superstitious in one’s reverence for the letter of the Bible, and that such superstition leads directly to heresy.  He explains the reticence of the New Testament on this point by shewing (in this Oration, cc. 26, 27) how God’s Self-Revelation to man has always been a gradual one; how the Old Testament revealed the Father clearly, with obscure hints about the Son; and the New Testament manifested the Son, but only hinted at the Godhead of the Spirit; but now, he says, the Spirit dwells among us, and allows us to recognize Him more clearly.  For it would not have been advisable, as long as the Godhead of the Father was not acknowledged, to proclaim that of the Son; and while the Deity of the Son was not yet accepted, to add another burden in that of the Holy Spirit.  Recognizing thus a Divine economy in the Self-Revelations of God, he was not averse to using a similar caution in his own dealings with weak or ill-instructed minds. In his Fifty-third Epistle, addressed to S. Basil, there is an amusing instance of his defence of this tolerant disposition, which S. Basil also displayed in dealing with minds of this class.  But yet when real necessity arose, he could speak out with perfect plainness on this subject; and he even incurred danger to life and limb from the violence of the opposing party.  He met their opposition by the clearest statements of the Catholic Dogma.  “Is the Spirit God?” he asks.  “Yes.”  “But is He consubstantial?”  “Yes, if He is God.”  (Orat. xxxi. 10.)  He appeals both to the Bible, and to the experience of the Christian life.  If the Spirit is not to be adored, how can He deify me in Baptism?  From the Spirit comes our new Birth; from the new Birth our new Life; and from the new Life our knowledge of the Dignity of Him from Whom it is derived (Ibid. C. 29).  He is, however, milder in his treatment of these heretics than of the strict Arians, both, as he says, because they approached more nearly to the Orthodox belief on the subject of the Son, and because their conspicuous piety of life shewed that their error was not altogether wilful.  In this Oration he shows that though the Name of God may not actually be given in the New Testament to the Holy Ghost, yet all the attributes of God are ascribed to Him, and that therefore the use of the Name is a matter of legitimate inference.  He carries on the argument in the Oration on Pentecost (No. XLI.  See the Introduction to that Oration in the present Volume).