Pope Liberius is the first pope in history not to be awarded sainthood by the Church. Once you read the controversy which embroiled his papacy, you will understand why.
Having had the freedom to practice the Faith for slightly more than a generation, the politics of the Church had become so entwined with that of the emperor of Rome that it had become impossible to separate the two.
Liberius became pope a month after the death of Pope Julius on April 12, 352. He took on a number of julius's advisors, including Vincentius of Capua, the papal legate to the Nicene Council some thirty years before. By this time, Vincentius must have been quite elderly. He had been there when the Arian heresy had been condemned.
The first recorded act extant is a letter Liberius wrote to the Emperor Constantius, an Arian supporter, the following year. Liberius had held a synod in Rome to discuss the heresy. He then wrote to the emperor who was wintering in Arles, Gaul, on the seacoast. Liberius asked for a council to be called at Aquileia, on the Adriatic coast, an easy access crossroads of the empire. The purpose was to discuss, further, the status of the heresy and Athanasius, the exiled bishop of Alexandria. Athanasius had remained staunchly orthodox throughout the years, holding that Jesus was of the same substance as the Father. The reason this made such a difference was that, if Jesus was not God, then he could not possible have redeemed us by His death. Now, you can understand why the question was so important!
Liberius sent the elderly Vincentius to the emperor with his missive. Constantius developed his own mini-council with predominantly Arian bishops and Vincentius was forced by the emperor to condemn Athanasius. To quote Constantius, "My will...is canon law".
Liberius rejected the condemnation. Constantius then insisted on the pope going to Milan where he tried to force a condemnation of Athanasius, and a support of the Arian heresy out of the pope. Liberius was able to hold his ground and, for his effort, was sent into exile in Thrace (Greece).
Being as the Church needed another pope, the bishops and nobility elected the archdeacon, Felix, to the position in 355. Felix, who ruled as Felix II, tended to side with the heresy. It became a difficult time as violence erupted in the city. Many hid and the ports and gates were watched so that no one could escape. People could not attend their churches for fear of actions against them. This lasted two years. In the meantime, Liberius was probably tortured and Athanasius was a wanted fugitive.
In 357, Constantius came to Rome. The Roman populace demanded that Liberius be returned to them. To cover his potential embarrassment at changing his own rulings, some questionable documents appeared to show that Liberius supported the condemnation and excommunication of Athanasius and the acceptance of the heresy. Whether Liberius actually wanted to write them or was forced, we may never know for sure.
Constantius, however, allowed Liberius back into Rome, assuming that he and Felix would jointly rule. Felix was ousted shortly and forced to retire to Porto.
The Arian question just wouldn't go away, so Constantius requested more councils almost every year. In 359, a new creed was proposed which skirted the "same substance" question completely. It quickly spread. Apparently, Liberius accepted it, possibly because of its ambiguity, possibly because he was vascillating and possibly because he was just so weak.
When Constantius died in 361, Liberius reversed himself to a point and condemned the new creed, proclaimed the divinity of Jesus and reversed the excommunication of Athanasius. He began to charitably accept moderate Arians who had condemned the creed and wanted full communion with the Church, again. The heresy began to splinter and disintegrate.
Liberius has been sanctioned for his vascillation. His excommunication of Athanasius, orthodox bishop of Alexandria, hurt his reputation for all time. Some people claim that this proves that popes are not infallible. However, an excommunication is not an ex cathedra statement for the whole Church and can not be taken as such. It is more a sadness of judgement and a show of weakness on the part of the pope and strength of character on the part of Bishop Athanasius. And, let us not forget, as Archbishop Lefebre has said, "It was the pope who recanted and Athanasius who was eventually canonized.”