6 But we have always maintained the birth existing out of time: we have taught that God the Son is God of the same nature with God the Father, not co-equal with the Unbegotten, for He was not Himself Unbegotten, but, as the Only-begotten, not unequal because begotten; that the Two are One, not by the giving of a double name to one Person, but by a true begetting and being begotten; that neither are there two Gods, different in kind, in our faith, nor is God solitary because He is one, in the sense in which we confess the mystery of the Only-begotten God: but that the Son is both indicated in the name of, and exists in, the Father, Whose name and Whose nature are in Him, while the Father by His name implies, and abides in, the Son, since a son cannot be spoken of, or exist, except as born of a father.
Further, we say that He is the living copy of the living nature, the impression of the divine seal upon the divine nature, so undistinguished from God in power and kind, that neither His works nor His words nor His form are other than the Father's: but that, since the image by nature possesses the nature of its author, the Author also has worked and spoken and appeared through His natural image.
Source: On the Trinity (New Advent)