The Arians, inasmuch as they assert the Son to be “of another substance,” plainly acknowledge substance in God. The only reason why they avoid the use of this term is that they will not, as Eusebius of Nicomedia has made it evident, confess Christ to be the true Son of God.
123. How can the Arians deny the substance of God? How can they suppose that the word “substance” which is found in many places of Scripture ought to be debarred from use, when they themselves do yet, by saying that the Son is “ἑτεροούσιος,” that is, of another substance, admit substance in God?
124. It is not the term itself, then, but its force and consequences, that they shun, because they will not confess the Son of God to be true [God]. For though the process of the divine generation cannot be comprehended in human language, still the Fathers judged that their faith might be fitly distinguished by the use of such a term, as against that of “ἑτεροούσιος,” following the authority of the prophet, who says: “Who has stood in the truth (substantia) of the Lord, and seen His Word?” Arians, therefore, admit the term “substance” when it is used so as to square with their blasphemy; contrariwise, when it is adopted in accordance with the pious devotion of the faithful, they reject and dispute against it.
125. What other reason can there be for their unwillingness to have the Son spoken of as “ὁ μοούσιος,” of the same substance, with the Father, but that they are unwilling to confess Him the true Son of God? This is betrayed in the letter of Eusebius of Nicomedia. “If,” writes he, “we say that the Son is true God and uncreate, then we are in the way to confess Him to be of one substance (ὁμοούσιος) with the Father.” When this letter had been read before the Council assembled at Nicæa, the Fathers put this word in their exposition of the Faith, because they saw that it daunted their adversaries; in order that they might take the sword, which their opponents had drawn, to smite off the head of those opponents' own blasphemous heresy.
126. Vain, however, is their plea, that they avoid the use of the term, because of the Sabellians; whereby they betray their own ignorance, for a being is of the same substance (ὁ μοούσιον) with another, not with itself. Rightly, then, do we call the Son “ὁ μοούσιος” (of the same substance), with the Father, forasmuch as that term expresses both the distinction of Persons and the unity of nature.
127. Can they deny that the term “οὐσία” is met with in Scripture, when the Lord has spoken of bread, that is, “ἐ πιούσιος,” and Moses has written “ὑμεῖς ἔσεσθέ μοι λαὸς περιούσιος”? What does “οὐσία” mean, whence comes the name, but from “οὖσα ἀεί,” that which endures for ever? For He Who is, and is for ever, is God; and therefore the Divine Substance, abiding everlastingly, is called οὐσία. Bread is ἐπιούσιος, because, taking the substance of abiding power from the substance of the Word, it supplies this to heart and soul, for it is written: “And bread strengthens man's heart.”
128. Let us, then, keep the precepts of our forefathers, nor with rude and reckless daring profane the symbols bequeathed to us. That sealed book of prophecy, whereof we have heard, neither elders, nor powers, nor angels, nor archangels, ventured to open; for Christ alone is reserved the peculiar right of opening it. Who among us dare unseal the book of the priesthood, sealed by confessors, and long hallowed by the testimony of many? They who have been constrained to unseal, nevertheless have since, respecting the deceit put upon them, sealed again; they who dared not lay sacrilegious hands upon it, have stood forth as martyrs and confessors. How can we deny the Faith held by those whose victory we proclaim?
Source: On the Christian Faith (De fide) (New Advent)