Concerning Lucifer and Eusebius, Bishops of the West. Eusebius with Athanasius the Great and Other Bishops collect a Council at Alexandria, and confirm the Nicene Faith by defining the Consubstantiality of the Spirit with the Father and the Son. Their Decree concerning Substance and Hypostasis
After the return of Athanasius, Lucifer, bishop of Cagliari in Sardinia, and Eusebius, bishop of Vercelli, a city of Liguria in Italy, returned from the upper Thebaïs. They had been condemned by Constantius to perpetual exile in that country. For the regulation and general systematizing of ecclesiastical affairs, Eusebius came to Alexandria, and there, in concert with Athanasius, to hold a council for the purpose of confirming the Nicene doctrines.
Lucifer sent a deacon with Eusebius to take his place in the council, and went himself to Antioch, to visit the church there in its disturbances.
A schism had been excited by the Arians then under the guidance of Euzoïus, and by the followers of Meletius, who, as I have above stated, were at variance even with those who held the same opinions as themselves. As Meletius had not then returned from exile, Lucifer ordained Paulinus bishop.
In the meantime, the bishops of many cities had assembled in Alexandria with Athanasius and Eusebius, and had confirmed the Nicene doctrines. They confessed that the Holy Ghost is of the same substance as the Father and the Son, and they made use of the term “Trinity.”
They declared that the human nature assumed by God the Word is to be regarded as consisting of not a perfect body only, but also of a perfect soul, even as was taught by the ancient Church philosophers. As the Church had been agitated by questions concerning the terms “substance” and “hypostasis,” and the contentions and disputes about these words had been frequent, they decreed, and, as I think, wisely, that these terms should not henceforth at the beginning be used in reference to God, except in refutation of the Sabellian tenet; lest from the paucity of terms, one and the same thing might appear to be called by three names; but that one might understand each by its peculiar term in a threefold way.
These were the decrees passed by the bishops convened at Alexandria. Athanasius read in the council the document about his flight which he had written in order to justify himself.
Source: Ecclesiastical History (New Advent)