It should be carefully noted that the question of the fall of Liberius is one that has been and can be freely debated among Catholics. No one pretends that, if Liberius signed the most Arian formulæ in exile, he did it freely; so that no question of his infallibility is involved. It is admitted on all sides that his noble attitude of resistance before his exile and during his exile was not belied by any act of his after his return, that he was in no way sullied when so many failed at the Council of Rimini, and that he acted vigorously for the healing of orthodoxy throughout the West from the grievous wound. If he really consorted with heretics, condemned Athanasius, or even denied the Son of God, it was a momentary human weakness which no more compromises the papacy than does that of St. Peter.https://www.newadvent.org/cathen/09217a.htm Liberius
Forged letters In the fragments of St. Hilary are embedded a number of letters of Liberius. Fragment IV contains a letter, "Studens paci", together with a very corrupt comment upon it by St. Hilary. The letter has usually been considered a forgery since Baronius (2nd ed.), and Duchesne expressed the common view when he said in his "Histoire ancienne de l'Église" (1907) that St. Hilary meant us to understand that it is spurious. But its authenticity was defended by Tillemont , and has been recently upheld by Schiktanz and Duchesne (1908), all Catholic writers. Hermant (cited by Coustant ), followed by Savio, believed that the letter was inserted by a forger in the place of a genuine letter, and he took the first words of St. Hilary's comment to be serious and not ironical: "What in this letter does not proceed from piety and from the fear of God ?" In this document Liberius is made to address the Arian bishops of the East, and to...