5 Their treason involves us in the difficult and dangerous position of having to make a definite pronouncement, beyond the statements of Scripture, upon this grave and abstruse matter. The Lord said that the nations were to be baptized in the Name of the Father, and of the Son, and of the Holy Ghost. The words of the faith are clear; the heretics do their utmost to involve the meaning in doubt. We may not on this account add to the appointed form, yet we must set a limit to their license of interpretation.
Since their malice, inspired by the devil's cunning, empties the doctrine of its meaning while it retains the Names which convey the truth, we must emphasise the truth which those Names convey. We must proclaim, exactly as we shall find them in the words of Scripture, the majesty and functions of Father, Son and Holy Spirit, and so debar the heretics from robbing these Names of their connotation of Divine character, and compel them by means of these very Names to confine their use of terms to their proper meaning.
I cannot conceive what manner of mind our opponents have, who pervert the truth, darken the light, divide the indivisible, rend the scatheless, dissolve the perfect unity. It may seem to them a light thing to tear up Perfection, to make laws for Omnipotence, to limit Infinity; as for me, the task of answering them fills me with anxiety; my brain whirls, my intellect is stunned, my very words must be a confession, not that I am weak of utterance, but that I am dumb. Yet a wish to undertake the task forces itself upon me; it means withstanding the proud, guiding the wanderer, warning the ignorant.
But the subject is inexhaustible; I can see no limit to my venture of speaking concerning God in terms more precise than He Himself has used. He has assigned the Names— Father, Son and Holy Ghost—which are our information of the Divine nature. Words cannot express or feeling embrace or reason apprehend the results of enquiry carried further; all is ineffable, unattainable, incomprehensible. Language is exhausted by the magnitude of the theme, the splendour of its effulgence blinds the gazing eye, the intellect cannot compass its boundless extent.
Still, under the necessity that is laid upon us, with a prayer for pardon to Him Whose attributes these are, we will venture, enquire and speak; and moreover— it is the only promise that in so grave a matter we dare to make— we will accept whatever conclusion He shall indicate.
Source: On the Trinity (New Advent)