3 Just now, I am sorry to say, those Arians, the Campenses, are trying to extort from me, a Roman Christian, their unheard-of formula of three hypostases. And this, too, after the definition of Nicæa and the decree of Alexandria, in which the West has joined. Where, I should like to know, are the apostles of these doctrines? Where is their Paul, their new doctor of the Gentiles? I ask them what three hypostases are supposed to mean. They reply three persons subsisting. I rejoin that this is my belief. They are not satisfied with the meaning, they demand the term. Surely some secret venom lurks in the words. “If any man refuse,” I cry, “to acknowledge three hypostases in the sense of three things hypostatized, that is three persons subsisting, let him be anathema.” Yet, because I do not learn their words, I am counted a heretic. “But, if any one, understanding by hypostasis essence, deny that in the three persons there is one hypostasis, he has no part in Christ.” Because this is my confession I, like you, am branded with the stigma of Sabellianism.
Source: Letters (New Advent)