Valentinianism was one of the major Gnostic Christian movements. Founded by Valentinus (b. c. 100 CE – d. c. 165 CE) in the 2nd century, its influence spread widely, not just within the Roman Empire but also from northwest Africa to Egypt through to Asia Minor and Syria in the east. Later in the movement's history, it broke into Eastern and a Western schools. The Valentinian movement remained active until the 4th century, declining after Emperor Theodosius I issued the Edict of Thessalonica in 380, which established Nicene Christianity as the state religion of the Roman Empire.
No evidence exists that Valentinus was labeled a heretic during his lifetime. Irenaeus of Lyons, who was the first patristic source to describe Valentinus's teachings—though likely incompletely and with a bias toward the time's proto-orthodox Christianity—did not finish his apologetic work Against Heresies until the latter 2nd century, likely sometime after Valentinus's death. The rapid growth of the Valentinian Gnostic movement—named eponymously after Valentinus—after his death prompted early Christian thought leaders, such as Irenaeus and later Hippolytus of Rome, to write apologetic works against Valentinus and Valentinianism, which conflicted with proto-orthodox theology. Because the proto-orthodox camp's leadership encouraged the destruction of Gnostic texts writ large, most textual evidence regarding Valentinian theology and practice comes from its critics—particularly Irenaeus, who was highly focused on refuting Valentinianism.
