Excerpts from the First Book, Translated by P. E. Pusey
FROM HIS TREATISE AGAINST DIODORE BISHOP OF TARSUS 
[Translated by P. E. Pusey]
of which the beginning is
Nought shall be ranked before the Truth by them at least who love it and are well skilled in uttering what pertains to it.
His words from the treatises against Diodore and Theodore: the beginning[ ]of the Treatise.
Nothing is valued before the Truth by them who love it and are skilful in speaking what pertains thereto : yet is it right (I say) that they who are thus minded and are zealous rightly to walk in the holy doctrines of the Church, should both guilelessly give heed to any who think and speak aright and not again, holden by reverence and love, commit themselves to those who write not without blame, in order that they be not blamed as calling evil good and good evil, sweet bitter and bitter sweet, and putting darkness for light, and light for darkness : but accomplishing rather that which is consonant to the Divine law (for Judge, it says, righteous judgement), consonant too to the wise Paul, Be ye wise hankers, prove all things : may accept that which is excellent, and keep far from what is not so. For it is absurd that irrational animals, should be instructed by the laws of nature, to know well what is good for them and what is not so: so that they make their food of those things in the field which have no harm in them, and leave those which do harm; and that WE who have understanding and right reason (for nature is wise and has perfectly the power of well examining each thing) should not rightly and without error examine the force of things written or spoken that we may honour with praise the things which are blameless, and turn aside from all which are unduly spoken and which step outside of the doctrines of the truth.
Albeit how ought not one who wanted to shew the difference of the properties, I mean of flesh and Godhead, to advance to this very point by such thoughts and words as were meet? For not the same as regards the quality which is inherent in each of the things named, are Body and the Unembodied, the flesh taken of human lump and the Word which beamed forth from the Essence of God the Father. Yet we must not therefore sever into two christs and sons the One Lord JESUS CHRIST.
But that we say that the Flesh of the Lord has been ensouled with reasonable soul, has been full often told by us, and now too no less do we affirm that it is no otherwise.
1. [a] Diodore, the "pupil (θρέμμα) of the blessed Silvanus" Bishop of Tarsus, the comrade of S. Flavian (afterward Bishop of Antioch) in toils for the Catholics of Antioch in their low estate through Arian oppression, visitor of S.Meletius Bishop of Antioch in his banishment in Armenia through these same Arians, teacher of S.Chrysostom, commentator on most of the Old and New Testament, present at the Second Council where he signed as Bishop of Tarsus, being then at the beginning of an Episcopate of about 13 years, and who died in the Unity of the Church, nevertheless fell into the error of so parting the two Natures in Christ as to speak of His Manhood as though it were a Man apart from the Son of God. S. Athanasius speaks as though he saw the germ of some such error; he says, "And He became man and did not come into man, for this it is necessary to know, lest perchance these irreligious men fall into this notion also, and beguile any into thinking that as in former times the Word was used to come into each of the Saints, so now He sojourned in a man, hallowing him also and manifesting Himself as in the others." against Arians, iii § 30 p. 442 O.T.
Of Diodore's writings little is preserved excepting some few citations in different writings of Severus. Even of S. Cyril's work these few fragments that survive seem almost entirely due to the Monophysite Controversy in the first half of the sixth century. The fragments are mainly preserved either by Severus of Antioch (chiefly in his work against John Grammaticus of Caesarea, but also in other works) and by John of Caesarea himself who appended a vast number of extracts of S. Cyril to his Apology for the Council of Chalcedon. Anastasius, referred to by the learned Cave, under Severus (Viae dux cap. 6 pp. 90, 92, ed. Ingolstadt, 1606) says of this John, "Then John of Caesarea Grammarian and very many more made defences for the synod (of Chalcedon) through truest extracts . . . Severus having looked into the compilations of the Caesarean and some others who compiled in behalf of the synod through very many extracts of Fathers and writers and demonstrations and proofs, first of all straightway wrote against John of Caesarea." Further on, Severus "laid down as a law to them [in Syria Egypt Alexandria and elsewhere] in the same book which is called Philalethes, that the Faith of Chalcedon frittered away 230 citations of holy Fathers in the defence which John of Caesarea made in its behalf." ib. p.96. In the MSS of John's Defence wnich have supplied many of these passages against Diodore and Theodore, they are numbered 181-196, Cave likewise refers to extracts of Severus' work against the Grammarian in the Catena on Old Testament Canticles edited by Anton. Caraffa. John of Caesarea signs in the fifth general Council as "John by the mercy of God Bishop of Caesarea of Palestine." t. vi. 218 Colet. He had been Bishop but a short time when the Council was called in A.D. 553, and probably, as Severus was dethroned in A.D. 536, the controversy had taken place before John was Bishop, which will account for his being usually styled John of Caesarea. Leontius of Jerusalem however cites at least once, from the Book of the same Severus against the Grammarian John Bishop of Caesarea. Apol. Conc. Chalc. in Gallandi, Bibl. Vett. Patrum xii. 736. The Lateran council similarly, The same Severus against John of holy memory Bishop of Caesarea of Palestine. Conc. vii. 324 Col. John of Caesarea's Defence of the Council of Chalcedon is extant in MS. in syriac (as Cardinal Mai tells us, in Cod. Vat. 140 written in the eighth Century), and in Greek in a late manuscript at Venice and at Cairo. Of the character of Diodore's writings the learned Tillemont who appears most marvellously to have made himself acquainted with every extant writing of every Father, says "We cannot be judges of this great difficulty [whether Diodore's writings were heretical] because we no longer have his writings which would need to be examined with great care, not stopping at culled passages." t. 8. 568 ed. 2. S. Cyril however who had access to them says of him, "One Diodore, being once as they say, an opponent of the SPIRIT, communicated with the Church of the Orthodox. This man having put off, as he deemed, the spot of the Macedonian heresy, fell into another infirmity. For he deemed and wrote that one son by himself is he who is of the seed of David, born of the holy Virgin ; another Son again by Himself the Word out of God the Father. But veiling the wolf under the sheep's fleece, he pretends to say One Christ, allotting the Name to the Word alone begotten out of God the Father, the Only-Begotten Son : and allotting it in the category of a grace, as himself says, he styles him too of the seed of David son, as united (he says) to the in truth Son: united, not as WE hold, but only as regards dignity, sway and equality of honour. His disciple Nestorius became, and darkened by Diodore's books, feigns" &c. Ep. 1 to Succensus 135 d e. Tillemont thinks that what S. Cyril says of Diodore having been a Macedonian, is not to be pressed, t. 8. 566.
Reference address : https://www.elpenor.org/cyril-alexandria/diodore-tarsus.asp