What happened 325 AD

In this conflict, Licinius was supported by the Gothic prince Alica. Due to the intervention of Flavia Julia Constantia, Constantine's sister and also Licinius' wife, both Licinius and his co-emperor Martinian were initially spared, Licinius being imprisoned in Thessalonica, Martinian in Cappadocia; however, both former emperors were subsequently executed. After his defeat, Licinius attempted to regain power with Gothic support, but his plans were exposed, and he was sentenced to death. While attempting to flee to the Goths, Licinius was apprehended at Thessalonica. Constantine had him hanged, accusing him of conspiring to raise troops among the barbarians.

Constantine established the principle that emperors could not settle questions of doctrine on their own but should instead summon general ecclesiastical councils for that purpose. His convening of both the Synod of Arles and the First Council of Nicaea indicated his interest in the unity of the Church and showcased his claim to be its head.