What is Councils of Sirmium?

The Councils of Sirmium were the five episcopal councils held in Sirmium in 347, 351, 357, 358 and finally in 375 or 378. The third—the most important of the councils—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.