Councils of Sirmium
The Council of Sirmium is the name primarily given to the third Council of Sirmium which marked a temporary compromise between Arianism and the Western bishops of the Christian church. In all, there were four councils held at Sirmium (now Sremska Mitrovica in the Vojvodina province of Serbia) between 347 AD and 359 AD of which at least three dealt primarily with the Arian controversy. All of these councils were held under the rule of Constantius II who was sympathetic to the Arians.
Background
Arianism was first put forward early in the fourth century by the Alexandrian presbyter Arius. It held that God was uniquely self-existent and immutable: consequently, Christ could not be God. The opponents of Arianism led by Athanasius of Alexandria claimed that the doctrine reduced Jesus to a demigod thus restoring polytheism as Jesus would still be worshipped. Further, it appeared to undermine the concept of redemption as only one who was truly God could reconcile man and God.
The First Council of Nicaea in 325 appeared to have settled the issue with Arius and his theology condemned and the Nicene Creed issued stating the Son was "of one substance with the father" (homoousion to Patri). However, Arians made a sustained effort to return to the church and to restore their beliefs after 325 with a prolonged theological dispute ensuing.
The First and Second Councils of Sirmium
Constantine the Great died in 337, leaving Constantius II, who favored Arianism, as emperor in the East and Constans, who favored Nicea, emperor in the West. A church council held at Antioch in 341 issued an affirmation of faith that excluded the homoousion clause; another council held at Sardica in 342 (now Sofia) achieved little.
Constantius, who had a residence in Sirmium, convened the first Council of Sirmium in 347. It opposed Photinus, the bishop of Sirmium, an anti-Arian who held a belief similar to Marcellus.
In 350, Constantius became the sole Emperor of both East and West, leading to a temporary strengthening of Arianism.
At the second Council of Sirmium in 351, Basil, bishop of Ancyra (now Ankara) and leader of the semi-Arians, had Photinus deposed. The semi-Arians held that the Son was "of similar substance" (homoiousios) to the Father. Sirmium II also drafted the Sixth Arian Confession, which was an expanded version of the Fourth Arian Confession and was consistent with the strength of the semi-Arians.
Third and Fourth Councils
Councils were held in Arles in 353 and Milan in 355, with Athanasius condemned at both. In 356, Athanasius began his third exile, and George was appointed bishop of Alexandria.
The third Council of Sirmium, in 357, was the high point of Arianism. The Seventh Arian Confession (Second Sirmium Confession) held that both homoousios (of one substance) and homoiousios (of similar substance) were unbiblical and that the Father is greater than the Son. (This confession was later known as the Blasphemy of Sirmium.)
A Council of Ancyra in 358, chaired by Basil, released a statement using the term homoousios. But the fourth Council of Sirmium, also in 358, proposed a vague compromise: it said simply that the Son was homoios ("like") the Father.
Triumph of the Nicene Creed
At two councils in 359 (Rimini in the West and Seleucia in the East), Constantius tried to impose Sirmium IV's homoios formula on the church.
A council in Constantinople in 361 also approved homoios ("like" the Father), stating that the Son was "like the Father who begot him," and rejected the use of ousia ("substance").
But at the death of Constantinus in 361, the Nicene party in the West, which affirmed homoousios ("of like substance"), consolidated its position. And in the East, at the death of Athanasius in 373, the Cappadocian Fathers (Basil of Caesarea, d. 379; his brother Gregory of Nyssa, d. 394; and Gregory Nazianzus, d. 390), took the lead in support of Nicene orthodoxy.
Theodosius I (emperor of the East, 379-395; sole emperor, 394-395) outlawed Arianism at his accession in 379.
Finally, in 381, the second ecumenical (worldwide) council of the church (the first had been Nicaea), the First Council of Constantinople, upheld the Nicene Creed.