Top Five Bible Passages Relevant to Orthodox Mariology

But why she is the only one who can be sinless?

Can you be specific? Which council and which decree?

I have soft copy accessible through Logos.com of the book by Hefele: A History of the Councils of the Church. Vol. 3 has the English translation of all statements of 16 sessions of Chalcedon Council. Session 1 was held in Oct 8 of 451 AD.

So far I am unable to find the text you quoted. Are you sure you gave me correct information?

Below is the text of the first session of Ephesus council, copied and pasted from Hefele: A History of the Councils of the Church. Vol. 3.

The text does mention three statement made by Athanasius but none of them refer to his work (Discourse 3 Against the Arians) you quoted. The council of Ephesus dealt with heretical teaching of Nestorius, not that of Arius (Arianism). Hence there is no reason why it quoted the above work of Athanasius.

SEC. 134. First Session, June 22, 431.—Presidency and Number of those present
There was still wanting one of the superior metropolitans (patriarchs), namely, John of Antioch. His bishops, he said, could not leave their dioceses before Renovation Sunday (Dominica in Albis), and then it would take them twelve days to travel to Antioch, and from thence to Ephesus thirty-nine, so that they could not arrive until some days after Pentecost. At last (just about Pentecost2) John came into the neighbourhood of Ephesus, and sent to Cyril a letter, which is still extant, full of friendliness, setting forth that the length of the road and the death of several of their horses had delayed the journey, but that nevertheless he was close at hand, and would appear at Ephesus in five or six days. In spite of this they waited sixteen days;4 and then two of the metropolitans of the patriarchate of Antioch, Alexander of Apamea and Alexander of Hierapolis, came and repeatedly declared that “John had bid them say that they were no longer to defer the opening of the Synod on his account, but, in case it should be necessary for him to delay longer, they were to do what was to be done.” From this they inferred that the Patriarch John was intending to avoid being personally present at the condemnation of his former priest and friend Nestorius. Cyril and his friends now decided therefore on the immediate opening of the Synod, and assembled for that purpose on the 28th day of the Egyptian month Payni (= June 22) 431, in the cathedral of Ephesus, which, with great suitableness for that assembly, was dedicated to the God-bearer, and named after her. On the day before, several bishops received a commission to go to Nestorius and invite him to the session, in order to give an account of his statements and doctrines. At first he replied, “I will consider it.” When, however, a second deputation, sent on the 22d of June by the Synod, then opening, came to him, his residence was, by command of Candidian, surrounded with troops, who prevented the bishops, by threats of blows, from entering, and Nestorius sent them word that “he would appear as soon as all the bishops were assembled.” The Synod now, for the third time, sent off some bishops to him; but these received no further answer, and were treated with insolence by the soldiers on guard in and around the house.
At an earlier period, sixty-eight Asiatic bishops, among whom were, in particular, Theodoret of Cyrus and the two above-mentioned metropolitans of Apamea and Hierapolis, in a letter to Cyril and Juvenal, had requested that they would be pleased to defer the opening of the Synod until the arrival of bishops from Antioch. Now, however, the imperial commissioner, Candidian, himself appeared in the place of assembly, in order to have the imperial decrees read, and to protest against the immediate opening of the Synod.3 His demand, that they should wait four days longer, remained disregarded, and the first solemn session began under the presidency of Cyril, who, as is expressly stated in the Acts, also represented the Pope. No fewer than 160 bishops were present from the beginning,5 and when (still at the first session) the document of deposition came to be subscribed, their number had increased to 198. Particularly were there twenty of those sixty-eight Asiatic bishops who had gone over to the side of the Synod, as is clear from a comparison of their names with the subscriptions of the synodal Acts.7 The first thing which was done at the Synod was the reading of the imperial letter of convocation to all the metropolitans (see above, p. 40). That they should begin with this had been proposed by the Presbyter Peter of Alexandria, who acted as senior notary during the whole Synod, and externally controlled the arrangement of the business. Thereupon Bishop Memnon of Ephesus pointed out that sixteen days had elapsed beyond the limit appointed for the opening; and Cyril explained that, even in accordance with the express command of the Emperor, they must without delay begin with the transactions respecting the faith. Thereupon reference was made to the first invitation sent to Nestorius on the previous day, and directly afterwards the second and third deputations, already referred to, were sent to him, and the reports of the bishops who had returned were received.2 As Nestorius decidedly declined to appear, they proceeded, on the motion of Juvenal, to an examination of the point of doctrine in question, and began by reading the Nicene Creed. They next proceeded to the reading of the second letter which Cyril, as we saw, had a long time before addressed to Nestorius, in which he had explained the doctrine of the hypostatic union of the Godhead and manhood in Christ (see above, p. 21). To the question of Cyril, whether this letter of his agreed with the contents of the Nicene Creed, all the bishops present answered, and among them 126 in short speeches still preserved (explanatory of their votes), in a manner entirely affirmative and consentient, and for the most part full of commendation for Cyril. It then came to the turn to read the letter which Nestorius had sent in answer to the letter of Cyril just mentioned (see above, p. 21), and after thirty-four bishops, in explaining their votes, had declared emphatically its non-agreement with the Nicene faith, all the bishops cried out together: “If any one does not anathematize Nestorius, let him be himself anathema: the true faith anathematizes him, the holy Synod anathematizes him. If any one has communion with Nestorius, let him be anathema. We all anathematize the letter and the doctrines of Nestorius. We all anathematize the heretic Nestorius and his adherents, and his impious faith and his impious doctrine. We all anathematize the impious (ἀσεβῆ) Nestorius,” and so forth.
Afterwards there were two other documents read, namely, the letter of Cœlestine and the Roman Synod (p. 25 f.), and that of S. Cyril and of the Alexandrian Synod to Nestorius; and the four clerics whom Cyril had sent to deliver that document to Nestorius were examined as to the result of their mission. They gave the information, with which we are already acquainted (see above, p. 34), that Nestorius had given them no answer at all. In order, however, to be quite clear as to whether he still persisted in his error, two bishops, Theodotus of Ancyra and Acacius of Melitene, who were personal friends of Nestorius, and had during the last three days been in habitual intercourse with him, and had endeavoured to convert him from his error, were questioned on oath respecting the matter. They announced that, unfortunately, all their efforts with him had been in vain.
In order, however, to submit the doctrinal point in question to a thorough investigation, and in the light of patristic testimony, at the suggestion of Flavian, Bishop of Philippi, a number of passages from the writings of the Fathers of the Church were now read, in which the ancient faith respecting the union of the Godhead and manhood in Christ was expressed. These were statements of the opinions of Peter, Bishop of Alexandria († 311), of Athanasius, Pope Julius I. († 352), Pope Felix I. († 274), Theophilus, Archbishop of Alexandria († 412), of Cyprian, Ambrose, Gregory of Nazianzus, Basil the Great, Gregory of Nyssa, Atticus of Constantinople († 426), and Amphilochius of Iconium († 394). All these early authorities knew nothing of the Nestorian separation of the Godhead and manhood, but, on the contrary, taught the true incarnation of the Logos. The venerable martyr, Peter, Bishop of Alexandria, said: “God the Logos was made flesh, and born of the Virgin’s womb;” but Athanasius used frequently and unhesitatingly the expression θεοτόκος, contested by Nestorius, and says: “As the flesh was born of the God-bearer Mary, so we say that HE (the Logos) was Himself born of Mary.” And in a second passage Athanasius strongly blames those who (exactly like Nestorius) say that “the suffering and crucified Christ is not God the Logos;” who distinguish between Christ and the Logos, and do not confess, and do not acknowledge, “that the Logos, inasmuch as HE assumed a body from Mary, was made man.” And in a third passage Athanasius teaches that “the Logos was in truth, in the full sense of the word (not θέσει = by adoption, external connection), made man, otherwise HE would not be our Redeemer.” In agreement with this Pope Julius said: “There are not two sons, one true who assumed the man, and another the man who was assumed by God, but an only-begotten God in heaven, and an only-begotten God on earth.” Even Pope Felix I., who lived more than a century and a half before Nestorius, rejected his error, when he wrote: “We believe in our Lord Jesus Christ, who was born of the Virgin Mary, that HE is the eternal Son and Word of God, and not a man assumed by God, distinct from that (Word). For the Son of God did not assume a man, so that this was distinct from Him, but HE, the perfect God, was at the same time perfect man, made flesh of the Virgin.” Less striking are the passages from Cyprian and Ambrose; but Gregory of Nazianzus is again quite explicit: “We do not sever the man from the Godhead, but declare both to be one and the same who at the beginning was not man, but God, and the only Son of God, before all time and without all corporeity, but who at the end of the ages assumed man for the sake of our salvation. We confess that HE is one and the same, divine and earthly, visible and invisible, and so forth, at the same time, so that through the whole man, who is at the same time God, the whole man, who has fallen into sin, is created anew.” The seven anathematisms, too, which Gregory of Nazianzus appends to this passage, are entirely anti-Nestorian, and the very first of them anathematizes those who do not call Mary θεοτόκος, and the fourth those who hold that there are two sons, the one eternal from the Father, and a second from Mary. Further, the passage selected from Basil sounds as if it had been written with reference to Nestorius; for it says: “The immeasurable and infinite God, without being capable of suffering (in Himself), by assuming flesh combated death, in order by His own suffering to deliver us from liability to suffering.” To the same effect, in fine, speak also Gregory of Nyssa, Atticus of Constantinople, Amphilochius of Iconium, and Theophilus of Antioch, “that God was born and died.”
In opposition to these patristic passages there were next read twenty passages, some longer and some shorter, from the writings of Nestorius, in which his fundamental views, which we have presented above connectedly, were expressed in separate parts and in concreto.
The last document which was produced at this first session was the letter of Capreolus, Archbishop of Carthage, in which he asks them, on account of the war in Africa (consequent upon the invasion of the Vandals), to excuse his own inability to be present, or to send any of his suffragan bishops. Besides, he said, the Emperor’s letter of invitation had not reached him until Easter 431, and thus too late; and Augustine, whose presence the Emperor specially wished, had died some time before. He (the archbishop) therefore sent only his deacon Bessula, and prayed the Synod to tolerate no novelties whatever in matters of religion. In this he does not refer expressly to Nestorius, but he unmistakeably indicates that he reckons his doctrines among the unauthorized novelties. The Synod gave its approval to this letter of the African bishop, and proceeded at once (the intermediate speeches are not known to us) to the condemnation of Nestorius. The sentence is as follows: ἡ ἁγία σύνοδος εἶπε· Πρὸς τοῖς ἄλλοις μήτε ὑπακοῦσαι βουληθέντος τοῦ ἀσεβεστάτου Νεστορίου τῇ παρʼ ἡμῶν κλήσει, μήτε μὴν τοὺς παρʼ ἡμῶν ἀποσταλέντας ἁγιωτάτους καὶ Θεοσεβεστάτους ἐπισκόπους δεξαμένου, ἀναγκαίως ἐχωρήσαμεν ἐπὶ τὴν ἐξέτασιν τῶν δυσσεβηθέντων αὐτῷ. Καὶ φωράσαντες αὐτὸν ἔκ τε τῶν ἐπιστολῶν, καὶ ἐκ τῶν συγγραμμάτων αὐτοῦ, καὶ ἐκ τῶν ἀρτίως παρʼ αὐτοῦ ῥηθέντων κατὰ τήνδε τὴν μητρόπολιν καὶ πρόσμαρτυρηθέντων, δυσσεβῶς φρονοῦντα καὶ κηρύττοντα, ἀναγκαίως κατεπειχθέντες ἀπό τε τῶν κανόνων, καὶ ἐκ τῆς ἐπιστολῆς τοῦ ἁγιωτάτου πατρὸς ἡμῶν καὶ συλλειτουργοῦ Κελεστίνου τοῦ ἐπισκόπου τῆς ‘Ρωμαίων ἐκκλησίας, δακρύσαντες πολλάκις, ἐπὶ τὴν σκυθρωπὴν κατʼ αὐτοῦ ἐχωρήσαμεν ἀπόφασιν. Ὁ βλασφημηθεὶς τοίνυν παρʼ αὐτοῦ κύριος ἡμῶν Ἰησοῦς Χριστὸς ὥρισε διὰ τῆς παρούσης ἁγιωτάτης συνόδου, ἀλλότριον εἶναι τὸν αὐτὸν Νεστόριον τοῦ ἐπισκοπικοῦ ἀξιώματος καὶ παντὸς συλλόγου ἱερατικοῦ; that is: “As, in addition to other things, the impious Nestorius has not obeyed our citation, and did not receive the holy bishops who were sent by us to him, we were compelled to examine his ungodly doctrines. We discovered that he had held and published impious doctrines in his letters and treatises, as well as in discourses which he delivered in this city, and which have been testified to. Urged by the canons (c. 74, Apostol.), and in accordance with the letter of our most holy father and fellow-servant Cœlestine, the Roman bishop, we have come, with many tears, to this sorrowful sentence against him, namely, that our Lord Jesus Christ, whom he has blasphemed, decrees by the holy Synod that Nestorius be excluded from the episcopal dignity, and from all priestly communion.”
As we have already remarked above, this judgment was in the first place subscribed by 198 bishops who were present. Some others afterwards took the same side, so that altogether over 200 subscribed.
The session had lasted from early in the morning into the night, and the assembled population of Ephesus waited the whole day to hear the decision. When this was at last known, there arose an universal rejoicing; they commended the Synod, and solemnly accompanied the members, particularly Cyril, with torches and censers to their houses. The city was also illuminated in many places. This is joyfully related by Cyril in one of the three letters which he despatched at that time to the members of his Church of Alexandria, and to the monks of Egypt.
On the next day the sentence which had been pronounced was sent to Nestorius himself in a very laconic edict. In the superscription he is called a new Judas, and in the text it is said briefly: “He must know that, on account of his impious doctrines and his disobedience to the canons (because he had not appeared in answer to the citations), he had been, on the 22d of June, in accordance with the ecclesiastical laws, deposed by the holy Synod, and expelled from the body of the clergy.”
In two other similarly curt letters of the same date, the one to the collective people, the other to the clergy of Constantinople, the Synod announced that which had been done, and required the latter to watch carefully over all the property of the Church of Constantinople, so as to be able to give an account of it to him who should, in accordance with the will of God and the indication (νεύματι) of the Emperor, become bishop of that city.
Cyril, as president of the Synod, wrote at greater length to his friends and agents in Constantinople, the Archimandrite Dalmatius and several (certainly Egyptian) bishops and priests, and related to them the whole course of the session, from the citation of Nestorius to his deposition, with the request that they would take care that no false rumours on the subject should go abroad. It was reputed that Count Candidian had already sent such false information (to the Emperor); whereas the Synod had not yet completed its full report (together with the Acts) to the Emperor.