11 But in this case the Word in very truth is God; the essence of the Godhead exists in the Word, and that essence is expressed in the Word's name. For the name Word is inherent in the Son of God as a consequence of His mysterious birth, as are also the names Wisdom and Power. These, together with the substance which is His by a true birth, were called into existence to be the Son of God; yet, since they are the elements of God's nature, they are still immanent in Him in undiminished extent, although they were born from Him to be His Son.
For, as we have said so often, the mystery which we preach is that of a Son Who owes His existence not to division but to birth. He is not a segment cut off, and so incomplete, but an Offspring born, and therefore perfect; for birth involves no diminution of the Begetter, and has the possibility of perfection for the Begotten. And therefore the titles of those substantive properties are applied to God the Only-begotten, for when He came into existence by birth it was they which constituted His perfection; and this although they did not thereby desert the Father, in Whom, by the immutability of His nature, they are eternally present.
For instance, the Word is God the Only-begotten, and yet the Unbegotten Father is never without His Word. Not that the nature of the Son is that of a sound which is uttered. He is God from God, subsisting through a true birth; God's own Son, born from the Father, indistinguishable from Him in nature, and therefore inseparable. This is the lesson which His title of the Word is meant to teach us. And in the same way Christ is the Wisdom and the Power of God; not that He is, as He is often regarded, the inward activity of the Father's might or thought, but that His nature, possessing through birth a true substantial existence, is indicated by these names of inward forces.
For an object, which has by birth an existence of its own, cannot be regarded as a property; a property is necessarily inherent in some being and can have no independent existence. But it was to save us from concluding that the Son is alien from the Divine nature of His Father that He, the Only-begotten from the eternal God His Father, born as God into a substantial existence of His own, has had Himself revealed to us under these names of properties, of which the Father, out of Whom He came into existence, has suffered no diminution.
Thus He, being God, is nothing else than God. For when I hear the words, And the Word was God, they do not merely tell me that the Son was called God; they reveal to my understanding that He is God. In those previous instances, where Moses was called god and others were styled gods, there was the mere addition of a name by way of title. Here a solid essential truth is stated; The Word was God. That was indicates no accidental title, but an eternal reality, a permanent element of His existence, an inherent character of His nature.
Source: On the Trinity (New Advent)