Council of Sirmium

The Council of Sirmium generally refers to the third of the four episcopal councils held in Sirmium between 357 AD and 359 AD. Specifically one was held in 357, one in 358 and one in 359. The third council marked a temporary compromise between Arianism and the Western bishops of the Christian church. At least two of the other councils also dealt primarily with the Arian controversy. All of these councils were held under the rule of Constantius II, who was sympathetic to the Arians.

Arianism was first put forward early in the 4th century by the Alexandrian presbyter Arius. It held that the Father is 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.

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.

...
Wikipedia