Accordingly, they first attack and condemn in his absence Athanasius, bishop of Alexandria, a holy man, who had been present as deacon at the Synod of Nicæa. For they added to the charges which false witnesses had heaped up against him, this one, that, with wicked intentions, he had received Marcellus and Photinus, heretical priests who had been condemned by a sentence of the Synod. Now, it was not doubtful as to Photinus that he had been justly condemned. But in the case of Marcellus, it seemed that nothing had then been found worthy of condemnation, and a belief in his innocence was above all strengthened by the animus of that party, inasmuch as no one doubted that those same judges were heretical by whom he had been condemned.
But the Arians did not so much desire to get these persons out of the way as Athanasius himself. Accordingly, they constrain the emperor to go so far as this, that Athanasius should be sent as an exile into Gaul. But ere long, eighty bishops, assembling together in Egypt, declare that Athanasius had been unjustly condemned. The matter is referred to Constantine: he orders bishops from the whole world to assemble at Sardes, and that the entire process by which Athanasius had been condemned, should be reconsidered by the council.
In the meantime, Constantine dies, but the Synod, called together while he was yet emperor, acquits Athanasius. Marcellus, too, is restored to his bishopric, but the sentence on Photinus, bishop of Sirmion, was not rescinded; for even in the judgment of our friends, he is regarded as a heretic. However, even this result chagrined Marcellus, because Photinus was known to have been his disciple in his youth. But this, too, tended to secure an acquittal for Athanasius, that Ursatius and Valens, leading men among the Arians, when they were openly separated from the communion of the Church after the Synod at Sardes, entering into the presence of Julius, bishop of Rome, asked pardon of him for having condemned the innocent, and publicly declared that he had been justly acquitted by the decree of the Council of Sardes.
Source: Sacred History (New Advent)