De Synodis
68 But if we attribute one substance to the Father and the Son to teach that there is a solitary personal existence although denoted by two titles: then though we confess the Son with our lips we do not keep Him in our hearts, since in confessing one substance we then really say that the Father and the Son constitute one undifferentiated Person. Nay, there immediately arises an opportunity for the erroneous belief that the Father is divided, and that He cut off a portion of Himself to be His Son. That is what the heretics mean when they say the substance is one: and the terminology of our good confession so gratifies them that it aids heresy when the word ὁμοούσιος is left by itself, undefined and ambiguous. There is also a third error. When the Father and the Son are said to be of one substance this is thought to imply a prior substance, which the two equal Persons both possess. Consequently the word implies three things, one original substance and two Persons, who are as it were fellow-heirs of this one substance. For as two fellow-heirs are two, and the heritage of which they are fellow-heirs is anterior to them, so the two equal Persons might appear to be sharers in one anterior substance. The assertion of the one substance of the Father and the Son signifies either that there is one Person who has two titles, or one divided substance that has made two imperfect substances, or that there is a third prior substance which has been usurped and assumed by two and which is called one because it was one before it was severed into two. Where then is there room for the Son's birth? Where is the Father or the Son, if these names are explained not by the birth of the divine nature but a severing or sharing of one anterior substance?
69. Therefore amid the numerous dangers which threaten the faith, brevity of words must be employed sparingly, lest what is piously meant be thought to be impiously expressed, and a word be judged guilty of occasioning heresy when it has been used in conscientious and unsuspecting innocence. A Catholic about to state that the substance of the Father and the Son is one, must not begin at that point: nor hold this word all important as though true faith did not exist where the word was not used. He will be safe in asserting the one substance if he has first said that the Father is unbegotten, that the Son is born, that He draws His personal subsistence from the Father, that He is like the Father in might, honour and nature, that He is subject to the Father as to the Author of His being, that He did not commit robbery by making Himself equal with God, in whose form He remained, that He was obedient unto death. He did not spring from nothing, but was born. He is not incapable of birth but equally eternal. He is not the Father, but the Son begotten of Him. He is not any portion of God, but is whole God. He is not Himself the source but the image; the image of God born of God to be God. He is not a creature but is God. Not another God in the kind of His substance, but the one God in virtue of the essence of His exactly similar substance. God is not one in Person but in nature, for the Born and the Begetter have nothing different or unlike. After saying all this, he does not err in declaring one substance of the Father and the Son. Nay, if he now denies the one substance he sins.
70. Therefore let no one think that our words were meant to deny the one substance. We are giving the very reason why it should not be denied. Let no one think that the word ought to be used by itself and unexplained. Otherwise the word ὁμοούσιος is not used in a religious spirit. I will not endure to hear that Christ was born of Mary unless I also hear, In the beginning was the Word, and the Word was God. I will not hear Christ was hungry, unless I hear that after His fast of forty days He said, Man does not live by bread alone. I will not hear He thirsted unless I also hear Whosoever drinks of the water that I shall give him shall never thirst. I will not hear Christ suffered unless I hear, The hour has come that the Son of man should be glorified. I will not hear He died unless I hear He rose again. Let us bring forward no isolated point of the divine mysteries to rouse the suspicions of our hearers and give an occasion to the blasphemers. We must first preach the birth and subordination of the Son and the likeness of His nature, and then we may preach in godly fashion that the Father and the Son are of one substance. I do not personally understand why we ought to preach before everything else, as the most valuable and important of doctrines and in itself sufficient, a truth which cannot be piously preached before other truths, although it is impious to deny it after them.
71. Beloved brethren, we must not deny that there is one substance of the Father and the Son, but we must not declare it without giving our reasons. The one substance must be derived from the true character of the begotten nature, not from any division, any confusion of Persons, any sharing of an anterior substance. It may be right to assert the one substance, it may be right to keep silence about it. You believe in the birth and you believe in the likeness. Why should the word cause mutual suspicions, when we view the fact in the same way? Let us believe and say that there is one substance, but in virtue of the true character of the nature and not to imply a blasphemous unity of Persons. Let the oneness be due to the fact that there are similar Persons and not a solitary Person.
72. But perhaps the word similarity may not seem fully appropriate. If so, I ask how I can express the equality of one Person with the other except by such a word? Or is to be like not the same thing as to be equal? If I say the divine nature is one I am suspected of meaning that it is undifferentiated: if I say the Persons are similar, I mean that I compare what is exactly like. I ask what position equal holds between like and one? I enquire whether it means similarity rather than singularity. Equality does not exist between things unlike, nor does similarity exist in one. What is the difference between those that are similar and those that are equal? Can one equal be distinguished from the other? So those who are equal are not unlike. If then those who are unlike are not equals, what can those who are like be but equals?
Source: On the Councils, or the Faith of the Easterns (New Advent)