When, after an interval of some time had elapsed, Athanasius, finding that Marcellus was by no means sound in the faith, suspended him from communion. And he had this degree of modesty, that, being censured by the judgment of so great a man, he voluntarily gave way. But though at a former period innocent, yet confessedly afterwards becoming heretical, it may be allowed to conclude that he was really then guilty when judgment was pronounced regarding him. The Arians, then, finding an opportunity of that kind, conspire to subvert altogether the decrees of the Synod of Sardes.
For a certain coloring of right seemed to be furnished them in this fact, that a favorable judgment had as unjustly been formed on the side of Athanasius, as Marcellus had been improperly acquitted, since now, even in the opinion of Athanasius himself, he was deemed a heretic. For Marcellus had stood forward as an upholder of the Sabellian heresy. But Photinus had already brought forward a new heresy, differing indeed from Sabellius with respect to the union of the divine persons, but proclaiming that Christ had his beginning in Mary.
The Arians, therefore, with cunning design, mix up what was harmless with what was blameworthy, and embrace, under the same judgment, the condemnation of Photinus, and Marcellus, and Athanasius. They undoubtedly did this with the view of leading the minds of the ignorant to conclude, that those had not judged incorrectly regarding Athanasius, who, it was admitted, had expressed a well-based opinion respecting Marcellus and Photinus. At that time, however, the Arians concealed their treachery; and not daring openly to proclaim their erroneous doctrines, they professed themselves Catholics.
They thought that their first great object should be to get Athanasius turned out of the church, who had always presented a wall of opposition to their endeavors, and they hoped that, if he were removed, the rest would pass over to their evil opinion. Now, that part of the bishops which followed the Arians accepted the condemnation of Athanasius with delight. Another part, constrained by fear and faction, yielded to the wish of the Arian party; and only a few, to whom the true faith was dearer than any other consideration, refused to accept their unjust judgment.
Among these was Paulinus, the bishop of Treves. It is related that he, when a letter on the subject was placed before him, thus wrote, that he gave his consent to the condemnation of Photinus and Marcellus, but did not approve that of Athanasius.
Source: Sacred History (New Advent)