pope from 352 to 366. He was elected on May 17, 352, to succeed Pope St. Julius I.

Liberius was pope during the turbulence caused by the rise of Arianism—a heresy teaching that Christ was not truly divine but was rather a created being. Liberius was pope under the Arian Roman emperor Constantius II, who opposed both the Council of Nicaea (Nicaea, Council of) (which had condemned Arianism) and Bishop St. Athanasius (Athanasius, Saint) of Alexandria (who was Arianism's most virulent opponent). Liberius's first act as pope was to write Constantius requesting a council at Aquileia, Italy, to discuss Athanasius, but the emperor independently effected Athanasius's condemnation. In 355 Liberius was one of the few bishops who refused to sign the condemnation, which had been imposed at Milan by imperial command upon all the Western bishops. Consequently, Constantius exiled Liberius to Beroea (modern Véroia, Greece), and the Arian archdeacon Felix (Felix (II)) (II) appropriated the papacy.

In late 357 Liberius went to Sirmium (modern Sremska Mitrovica, Serbia). Supposedly dejected, he agreed to sign certain unorthodox formulas that served to emasculate the Nicene Creed (the Creed had implicitly disavowed Arianism). Liberius also agreed to sever relations with Athanasius and submitted to the authority of the emperor. But Constantius recalled him to Rome, where he returned in 358, joyfully received by the Roman Christians. Felix fled to Porto, Italy, but Constantius decreed that Felix and Liberius should corule.

Although this imperial order was disregarded, Liberius's prestige was impaired. Neither he nor Felix was invited to the great council (Ariminum, Council of) that met at Rimini, Italy, in 359 to terminate the Arian crisis. This temporary humiliation prevented the papacy's involvement in the council's capitulation to imperial despotism and in its compromise with heresy. After Constantius's death in 361, Liberius annulled the decrees of Rimini. In 362, with his authority renewed, he received some Eastern bishops and had them profess the Nicene faith and anathematize the formulary of Rimini. The curious phenomenon of the papacy's double occupation ended when Felix died in 365.