4 If you think fit enact a decree; and then I shall not hesitate to speak of three hypostases. Order a new creed to supersede the Nicene; and then, whether we are Arians or orthodox, one confession will do for us all. In the whole range of secular learning hypostasis never means anything but essence. And can any one, I ask, be so profane as to speak of three essences or substances in the Godhead? There is one nature of God and one only; and this, and this alone, truly is. For absolute being is derived from no other source but is all its own.
All things besides, that is all things created, although they appear to be, are not. For there was a time when they were not, and that which once was not may again cease to be. God alone who is eternal, that is to say, who has no beginning, really deserves to be called an essence. Therefore also He says to Moses from the bush, “I am that I am,” and Moses says of Him, “I am has sent me.” As the angels, the sky, the earth, the seas, all existed at the time, it must have been as the absolute being that God claimed for himself that name of essence, which apparently was common to all.
But because His nature alone is perfect, and because in the three persons there subsists but one Godhead, which truly is and is one nature; whosoever in the name of religion declares that there are in the Godhead three elements, three hypostases, that is, or essences, is striving really to predicate three natures of God. And if this is true, why are we severed by walls from Arius, when in dishonesty we are one with him? Let Ursicinus be made the colleague of your blessedness; let Auxentius be associated with Ambrose. But may the faith of Rome never come to such a pass!
May the devout hearts of your people never be infected with such unholy doctrines! Let us be satisfied to speak of one substance and of three subsisting persons— perfect, equal, coeternal. Let us keep to one hypostasis, if such be your pleasure, and say nothing of three. It is a bad sign when those who mean the same thing use different words. Let us be satisfied with the form of creed which we have hitherto used. Or, if you think it right that I should speak of three hypostases, explaining what I mean by them, I am ready to submit.
But, believe me, there is poison hidden under their honey; the angel of Satan has transformed himself into an angel of light. They give a plausible explanation of the term hypostasis; yet when I profess to hold it in the same sense they count me a heretic. Why are they so tenacious of a word? Why do they shelter themselves under ambiguous language? If their belief corresponds to their explanation of it, I do not condemn them for keeping it. On the other hand, if my belief corresponds to their expressed opinions, they should allow me to set forth their meaning in my own words.
Source: Letters (New Advent)