Hilary had taken no part in the Synod held at Ancyra in the spring of a.d. 358, but he had been made acquainted with its decisions and even with the anathemas which thelegates of that Synod concealed at Sirmium. He saw that these decisions marked an approach. The horror which was felt at the Sirmian Blasphemia by those Eusebians whose only objection to the Nicene faith was that they did not understand it, augured well for the future. At the same time the majority of the Eastern bishops were deliberately heretical. It was natural that Hilary should be anxious about the episcopate of the West.
He had been in exile about three years and had corresponded with the Western bishops. From several quarters letters had now ceased to arrive, and the fear came that the bishops did not care to write to one whose convictions were different to their own. Great was his joy when, at the end of the year 358, he received a letter which not only explained that the innocent cause of their silence was ignorance of his address, but also that they had persistently refused communion with Saturninus and condemned the Blasphemia.
Early in 359 he dispatched to them the Liber de Synodis. It is a double letter, addressed to Western bishops, but containing passages intended for Orientals, into whose hands the letter would doubtless come in time. Hilary had recognized that the orthodox of the West had kept aloof from the orthodox of the East, firstly from ignorance of events, secondly from misunderstanding of the word o9moon/sioj, and thirdly from the feelings of distrust then prevalent. These facts determined the contents of his letter.
He begins with an expression of the delight he experienced on receiving the news that the Gallican bishops had condemned the notorious Sirmian formula. He praises the constancy of their faith.
He then mentions that he has received from certain of their number a request that he would furnish them with an account of the creeds which had been composed in the East. He modestly accedes to this request beseeching his readers not to criticise his letter until they have read the whole letter and mastered the complete argument. His aim throughout is to frustrate the heretic and assist the Catholic.
In the first or historical division of the letter he promises a transcription, with explanations, of all the creeds drawn up since the Council of Nix!a. He protests that he is not responsible for any statement contained in these creeds, and leaves his readers to judge of their orthodoxy.
The Greek confessions had already been translated into Latin, but Hilary considered it necessary to give his own independent translations, the previous versions having been half-unintelligible on account of their slavish adherence to the original.
The historical part of the book consists of fifty-four chapters (c. 10-63). It begins with the second Sirmian formula, and the opposing formula promulgated at Ancyra in a.d. 358. The Sirmian creed being given in c. 10, Hilary, before proceeding to give the twelve anathemas directed against its teaching by the bishops who assembled at Ancyra, explains the meaning of essentia and substantia. Concerning the former he says, Essentia est res quae est, vel ex quibus est, et quae in eo quod maneat subsistit. This essentia is therefore identical with substantia, quia res quae est necesse est subsistat in sese. The Ancyran anathemas are then appended, with notes and a summary.
In the second division (c. 29-33) of the historical part, Hilary considers the Dedication creed drawn up at Antioch in a.d. 341. He interprets it somewhat favourably. After stating that the creed is perhaps not sufficiently explicit in declaring the exact likeness of the Father and the Son, he excuses this inadequacy by pointing out that the Synod was not held to contradict Anomcean teaching, but teaching of a Sabellian tendency. The Complete similarity of the Son's essence to that of the Father appears to him to be guarded by the phrase Deum de Deo, totum ex toto.
The third division (c. 34-37) contains the creed drawn up by the Synod, or Cabal Synod, which met at Philippopolis in a.d. 343. Hilary does not discuss the authority of the Synod; it was enough for his purpose that it was composed of Orientals, and that its language emphatically condemns genuine Arianism and asserts the Son is God of God. The anathema which the creed pronounces on those who declare the Son to have been begotten without the Father's will, is interpreted by Hilary as an assertion that the eternal Birth was not conditioned by those passions which affect human generation.
The fourth division (c. 38-61) contains the long formula drawn up at Sirmium in a.d. 351 against Photinus. The twenty-seven anathemas are then separately considered and commended. The two remaining chapters of the historical part of the work include a reflection on the many-sided character of these creeds both in their positive and negative aspects. God is infinitus et immensus, and therefore short statements concerning His nature may often prove misleading. The bishops have used many definitions and phrases because clearness will remove a danger. These frequent definitions would have been quite unnecessary if it had not been for the prevalence of heresy. Asia as a whole is ignorant of God, presenting a piteous contrast to the fidelity of the Western bishops.
The theological part of the work opens in c. 64 with Hilary's exposition of his own belief. He denies that there is in God only one personality, as he denies that there is any difference of substance. The Father is greater in that He is Father, the Son is not less because He is Son. He asks his readers to remember that if his words fall short, his meaning is sound. This done, he passes to discuss the meaning of the word o9oou/sion. Three wrong meanings may be attributed to it. Firstly, it may be understood to deny the personal distinctions in the Trinity. Secondly, it may be thought to imply that the divine essence is capable of division. Thirdly, it may be represented as implying that the Father and the Son both equally partake of one prior substance. A short expression like o9oo/sioj must therefore receive an exact explanation. A risk is attached to its use, but there is no risk if we understand it to mean that the Father is unbegotten and the Son derives His being from the Father, and is like Him in power, and honour, and nature. The Son is subordinate to the Father as to the Author of His being, yet it was not by a robbery that He made Himself equal with God. He is not from nothing. He is wholly God. He is not the Author of the divine life, but the Image. He is no creature, but is God. Not a second God, but one God with the Father through similarity of essence. This is the ideal meaning of o9oiou/sioj, and in this sense it is not an error to assert, but to deny, the consubstantiality.
Hilary then makes a direct appeal to the Western bishops. They might forget the contents of the word while retaining the sound, but provided that the meaning was granted, what objection could be made to the word? Was the word o9oiou/sion free from all possible objections? Hilary (c. 72-75) shews that really like means really equal. Scripture is appealed to as proving the assertion that the Son is both like God and equal to God. This essential likeness can alone justify the statement that the Father and the Son are one. It is blasphemous to represent the similarity as a mere analogy. The similitude is a similitude of proper nature and equality. The conclusion of the argument is that the word o9oiou/sioj, if understood, leads us to the word o9oou/sioj which helps to guard it, and that it does not imply any separation between the Persons of the Trinity.
The saint now turns to the Eastern bishops, a small number of whom still remained faithful. He bestows upon them titles of praise, and expresses his joy at the decisions they had made, and at the Emperor's repudiation of his former mistake. With Pauline fervour Hilary exclaims that he would remain in exile all his life, if only truth might be preached.
Then, in a chapter which displays alike his knowledge of the Bible and his power of refined sarcasm, he unveils his suspicions concerning Valens and Ursacius. He doubts whether they could have been so inexperienced as to be ignorant of the meaning of the word o9moou/sioi when they signed the third Sirmian Creed. Furthermore he is obliged to point out a defect in the letter which the Oriental bishops wrote at the Synod of Ancyra. The word o9moou/sion is there rejected. The three grounds for such rejection could only be that the word was thought to imply a prior substance, or the teaching of Paul of Samosata, or that the word was not in Scripture. The first two grounds were only illusions, the third was equally fatal to the word o9moiou/sion. Those who intelligibly maintained o9moou/sion or o9oiou/sion , meant the same thing and condemned the same impiety (c. 82). Why should any one wish to decline the word which the Council of Nicaea had used for an end which was unquestionably good ? The argument is enforced by the insertion of the Nicene Creed in full. True, the word or o9oou/sion is quite capable of misconstruction. But the application of this test to the difficult passages in the Bible would lead to the chaos of all belief. The possible abuse of the word does not abolish its use. The authority of the eighty bishops who condemned the Samosatene abuse of it does not affect the authority of the three hundred and eighteen who ratified its Nicene meaning Hilary adds a statement of great importance. Before he was acquainted with the term he had personally believed what it implied. The term has merely invigorated his previous faith (c. 88, cf. c. 9I). In other words, Hilary tells his contemporaries and tells posterity that the word o9moou/sion, is Scripture because it is the sense of Scripture, and is truly conservative because it alone adequately preserves the faith of the fathers. The argument is interwoven with a spirited appeal to the Eastern bishops to return to that faith as expressed at Nicaea.
The last chapter (c. 92) is addressed to the Western bishops. It modestly defends the action of Hilary in writing, and urges a corresponding energy on the part of his readers. The whole concludes with a devout prayer.
The Liber de Synodis, like other works in which Catholicism has endeavoured to be conciliatory, did not pass unchallenged. It satisfied neither the genuine Arian nor the violently orthodox. The notes or fragments which we call Hilary's Apology throw light upon the latter fact. Hilary has to explain that he had not meant that the Eastern bishops had stated the true faith at Ancyra, and tells his Lord and brother Lucifer that it was against his will that he had mentioned the word o9moiou/sion. We must ourselves confess that Hilary puts an interpretation on the meaning of the Eastern formulae which would have been impossible if he had written after the Synod of Ariminum. Speaking when he did, his arguments were not only pardonable but right.