The doctrine had in view for enforcement is corroborated by the truth that the Son is the Word of the Father— the Word, not in the sense in which we understand the term, but a living and active Word. This being so, we cannot deny Him to be of the same Will, Power, and Substance with the Father.
73. To return, however, to what we had in hand before, and finish the task set before us. The Son, as the Word, carries out His Father's Will. Now, a word, as we understand and use it, is an utterance. There are syllables and sounds, which, however, are not at variance with the thought of our mind, and what we apprehend and are affected by inwardly we give token of by the testimony of the spoken word, which, as it were, works [for us]. But the words we speak have no direct efficacy in themselves, it is the Word of God alone, which is neither an utterance, nor an “inward concept,” as they call it, but works efficaciously, is living, and has healing power.
74. Would you know what is the nature of the Word— hear the Scriptures. “For the Word of God is living and mighty, yea, working effectually, sharp and keener than any the sharpest sword, piercing even to the sundering of soul and spirit, of limbs and marrow.”
75. Do you hear, then, the Word of God, and will you separate Him from the Father's Will and Power? You hear Him called the living Word, the healing Word— seek not then to compare Him with the word of our mouth; for if the word we utter, though it have not eyes to see, nor ears to hear, yet speaks, and still the knowledge of what it speaks is wrought by virtue of hidden mysteries of man's nature, how can he escape the charge of blasphemy, who requires that some sort of bodily vision and hearing shall go along with the Godhead in the Word of God, and thinks that the Son can do nothing of Himself, save what He shall have seen the Father doing, though (as we have said) there is in the Father, Son, and Holy Spirit the same Will, both to do and not to do, and the same Power, by reason of unity in the same substance.
76. But if, though men are, as a rule, different in respect of their thoughts and feelings, they yet agree as to the meaning of a single proposition, what ought we to think as concerning the Father and the Son of God, seeing that in the Substance of the Godhead there is that is imitated by human love?
77. Let us, however, suppose— as our adversaries would have it— that the Son does, as it were, copy the pattern of that which He has seen His Father doing. But even this, we must confess, means that He is of the same substance, for none can completely imitate the working of another, unless he be one with him in the same nature.
Source: On the Christian Faith (De fide) (New Advent)