351–360

The Triumph of Homoian Arianism under Constantius II: the Councils of Sirmium and the "Dated Creed"

Having become sole ruler in the 350s, Constantius II openly adopted a pro-Arian policy: "My will is the church's canon". Athanasius was exiled, and troops surrounded the church where he was serving.

The councils of Sirmium (351, 357) worked out compromise formulas. In 357 a council banned the non-biblical words "substance", "of one substance" and "of like substance". It attributed the suffering on the Cross to the assumed humanity rather than the Godhead. The attempt failed, driving moderate semi-Arians toward the Nicenes.

The fourth Sirmian formula – the "Dated Creed" of May 22, 359 – avoided the word "substance" altogether, calling the Son merely "like the Father". It was imposed on western bishops at Ariminum and eastern bishops at Seleucia, both in 359. Its variant was adopted at Nike in Thrace. Even the aged Hosius of Cordoba and Pope Liberius signed Arian formulas.

In 360 Homoian doctrine became the empire's official confession for two decades. In Jerome's words, "the whole world groaned and marveled to find itself Arian".