In 358 a rupture occurred between the anti-Nicene party. The Homoiousians became distinct from the Homoeans, and both condemned the Anomeanism of Aeitius and Eunomious who dared to say that the Son was of an “unlike substance” than the Father. Following this another ecumenical council was called by Constantius with hopes of finally achieving a new creed. The original desire was to hold the council at Nicomedia, but an earthquake in the area raised fears of the judgment of God. Another plan was to have the council meet at Nike, with the intention of creating a new creed of Nice (Nicaea). This too was not to be.
A council met at Sirmium in 358. It was lead by Basil of Ancyra, Valens, Ursacius, and George of Alexandria, the Homoean rival bishop to Athanasius. The council adopted the Dated Creed of 357 (“the blasphemy”) and greatly influenced the proceedings that would come at Seleucia. There was still no empire-wide agreement though, and thus Constantius still desired an ecumenical council. The eventual result was a calling of joint councils with Easterners meeting at Seleucia in 359 and Westerners meeting in Arminium in 360. Much like the earlier attempt at Serdica, the Easterners were mostly anti-Nicene and the Westerners were mostly pro-Nicene.
The Easterners at Seleucia adopted the earlier conclusion of Sirmium in 358, however they omitted the phrase “in all things” after “like” in reference to Jesus’ relationship to the Father. All ousia language was forbidden, and thus a decidedly Homoean creed was composed. Hilary of Poitiers was present at this council, as he was exiled to the East. He refused to participate in what he no doubt considered to be a rejection of the faith; however, he was called upon to defend the orthodoxy of the Gauls. In this he was successful, but this success would be bittersweet as any acceptance by Homoeans immediately cast doubt on true Nicene orthodoxy.
The Western council at Arminium was more complex. Valens appeared and unveiled the Dated Creed which Seleucia had adopted, and it was rejected out of hand. Indeed the delegates forbade any additions to the Creed of Nicaea as well as the deletion of ousia. Some delegates were sent to Nike where they were influenced, upon news of the results of Seleucia, to remove ousia from the creed. The majority at Arminium protested, and most of the early church historians defend the orthodoxy and intentions of the delegates there. Arminium sent 10 delegates to the emperor in Constantinople with a letter confessing the Nicene faith and an anathema of Arius and his supporters. The minority party, as well as the Eastern Bishops from Seleucia also sent 10 delegates, who, upon separation from the rest of the orthodox bishops, were able to persuade the pro-Nicene delegates to again adopt the Seleucian conclusion.
When the delegates from Arminium arrived in Constantinople, Constantius was already prepared to adopt the conclusion of Seleucia. The conclusion was a Synod of Constantinople in 360 where Constantius forcefully enacted the modified Dated Creed. This effectively made a Homoean creed the official confession of the imperial church. All use of ousia was forbidden, and in the eyes of the pro-Nicene, Arianism had triumphed.