24 And so God Only-begotten, containing in Himself the form and image of the invisible God, in all things which are properties of God the Father is equal to Him by virtue of the fullness of true Godhead in Himself. For, as we have shown in the former books, in respect of power and veneration He is as mighty and as worthy of honour as the Father: so also, inasmuch as the Father is always Father, He too, inasmuch as He is the Son, possesses the like property of being always the Son.
For according to the words spoken to Moses, He Who is, has sent Me unto you, we obtain the unambiguous conception that absolute being belongs to God; since that which is, cannot be thought of or spoken of as not being. For being and not being are contraries, nor can these mutually exclusive descriptions be simultaneously true of one and the same object: for while the one is present, the other must be absent. Therefore, where anything is, neither conception nor language will admit of its not being.
When our thoughts are turned backwards, and are continually carried back further and further to understand the nature of Him Who is, this sole fact about Him, that He is, remains ever prior to our thoughts; since that quality, which is infinitely present in God, always withdraws itself from the backward gaze of our thoughts, though they reach back to an infinite distance. The result is that the backward straining of our thoughts can never grasp anything prior to God's property of absolute existence; since nothing presents itself, to enable us to understand the nature of God, even though we go on seeking to eternity, save always the fact that God always is.
That then which has both been declared about God by Moses, that of which our human intelligence can give no further explanation; that very quality the Gospels testify to be a property of God Only-begotten; since in the beginning was the Word, and since the Word was with God, and since He was the true Light, and since God Only-begotten is in the bosom of the Father, and since Jesus Christ is God over all.
Source: On the Trinity (New Advent)