In 357 Constantius’ program of pushing for theological consensus in the West came to a significant head at the council of Sirmium. Actually, it is contested whether there was any kind of synod per se, with T.D. Barnes suggesting that it was actually a much smaller gathering that produced this creed, and that the only actual council was the earlier 351 council.
Regardless, we do have a text of a creed produced at this time, of which a Latin version is given in Hilary, De Synodis 11, and a Greek translation in Athanasius De Synodis 28 as well as Socrates HE 2.29.
My translation follows Hilary’s Latin, in accordance with the note in Athanasius that the creed was composed in Latin, but I have included some points of comparison with the Greek.
Since there was thought to be some debate concerning the faith, all things have been carefully handled and discussed at Sirmium, with the most holy brothers and our fellow-bishops Valens, Ursacius, and Germinius being present. It is known that there is one God, almighty and Father, just as is believed throughout the whole world: and his only son Jesus Christ the Lord our Saviour, begotten before the ages from that very one. Yet it is neither possible nor proper to declare Two Gods; because the Lord himself says, I will go to my Father and your Father, to my God and your God, for this reason there is one God of all things, just as the Apostle taught. Is he God of the Jews alone? Is he not also of the Gentiles? Indeed, also of the gentiles. Since indeed it is one God, who justifies the circumcision by faith, and the uncircumcision through faith. But in all other matters they agree, nor were they able to have any difference. Insofar as some have been disturbed by ‘substantia’, which is termed οὐσία in Greek, that is (to be understood more precisely), ὁμοουσίον, or what is termed ὁμοιούσιον, no mention ought to be made of these whatsoever. And neither is anything to be said about them from the cause and rationale that they are not contained in the divine Scriptures, and that they are above the knowledge that pertains to humanity, nor is anyone able to explain the birth of the Son, concerning which it is written, Who shall explain his generation? However it is clear that the Father alone knows how he generated his own Son, and the Son who he was generated from the Father. There is no ambiguity, that the Father is greater. It is doubtful to none, that the Father in honour, dignity, splendour, majesty, and in the very name of Father, is greater than the Son, the Son himself testifying He that sent me is greater than I, and that this is the catholic [dogma], no one is ignorant, that there are two persons of Father and Son, the Father greater, the Son subordinate with all those things which the Father has subordinated to him [i.e. to the Son]. That the Father has no beginning, is invisible, immoral, impassible. But that the Son is born of the Father, God from God, light from light. The generation of which Son, as was said before, no-one is able to know except his own Father. But that this Son of God, our lord and God, as is read, took flesh or body, that is , humanity, from the womb of the Virgin Mary, as the Angel announced, as all the Scriptures teach, and especially the Apostle, the teacher of the Gentiles himself, that he took humanity from the Virgin Mary, through which he suffered. This is the conclusion and verification of the whole faith, that the Trinity is to be always preserved, as we read in the Gospel, Go and baptise all nations in the name of the Father and of the Son and of the Holy Spirit, whole and perfect is the number of the Trinity. The Paraclete, the Spirit, is through the Son, who was sent, came according to promise, to instruct, teach and sanctify the apostles and all believers.
The creed of Sirmium 357 is often taken to be another major turning point in the debates of the 4th century. Our accounts of what actually occurred are scarce and confused. It seems that the creed was composed primarily by Ursacius and Valens. Hilary attributes it also to Ossius, but he was close to 100, so his active participation may be slightly doubted. Some call this an Anomoian creed, but it is not quite that far gone. Far better to identify this as the emergence of a full-fledged Homoian position. This is seen not merely in the careful avoidance of ousia and homo(i)ousion language, but direct opposition to it coupled with forbiddance of the terminology, on the basis of it being unscriptural and that the manner of generation is unknowable, and so no longer to be discussed. This isn’t theologising, it’s the proscription of theologising. The unity of Father and Son is reduced to agreement/harmony/συμφωνία, and the Son is strongly subordinated to the Father. John 14:28 is cited, but no attempt is made to reconcile this in anyway with John 10:30. The mention of the Spirit is incredibly brief, echoing the purpose clause of the 4th Antiochene, but little else.
The effect of Sirmium 357 is twofold. On the one hand we see the emergence of a Homoian ‘faction’ or ‘alliance’ with figures such as Acacius of Caesarea and Eudoxius of Antioch (bishop of Antioch, later of Constantinople) taking the lead, as evidenced by the subsequence Council of Antioch in 357 or 358, where Eudoxius and Acacius ratify and affirm the Sirmium text. The Homoian faction was not particularly stable, and Western and Eastern adherents headed in slightly different directions. Furthermore the Eastern Homoians were not themselves able to hold a coherent platform, as seen with the emergence of Aetius and the genuine Anomoian/Heterousian position in the early 360s.
On the other hand you see the emergence of reaction against Sirmium 357, not least in Hilary’s decision to write De Synodis, but also the response of George of Laodicea and Basil of Ancyra, of which we will treat shortly in association with the Council of Ancyra 358.
 T.D. Barnes, Athanasius and Constatnius: Theology and Politics in the Constantinian Empire Cambrdige, Mass.: HUP, 1992, pp231-32.
 cf. μονογενῆ in Ath.
 ex ipso, ἐξ αὐτοῦ. If the Latin were truly reflexive, it would have to be se, and the Greek would have to follow.
 John 20:17
 Romans III, 29-30. Latin has present where Greek has future for justify.
 convenerunt, cf. συμφωνεῖ
 Isaiah 53:8. enarrare is the Latin, διηγέομαι in the Greek.
 John 14:28.
 natum, cf. γεγεννῆσθαι
 cf. Luke 1:31
 quem, referring back to hominem; the ‘man’ that Jesus’ took to himself was the means through which he suffered.
 I have preferred to translate homo as humanity, rather than ‘man’, since what is in view is clearly the assumption of human nature.
 Mt 28:19
 Hanson, The Search, 343ff., Simonetti, La Crisi, 227ff.
 Sozomen iv.12-15
 Ayres covers some of this in Nicaea and its Legacy, pp137-9.