8 We have now exchanged the perils of a harbourless coast for the storms of the open sea. We can neither safely advance nor safely retreat, yet the way that lies before us has greater hardships than that which lies behind. The Father is what He is, and as He is manifested, so we must believe. The mind shrinks in dread from treating of the Son; at every word I tremble lest I be betrayed into treason. For He is the Offspring of the Unbegotten, One from One, true from true, living from living, perfect from perfect; the Power of Power, the Wisdom of Wisdom, the Glory of Glory, the Likeness of the invisible God, the Image of the Unbegotten Father.
Yet in what sense can we conceive that the Only-begotten is the Offspring of the Unbegotten? Repeatedly the Father cries from heaven, This is My beloved Son in Whom I am well pleased . It is no rending or severance, for He that begot is without passions, and He that was born is the Image of the invisible God and bears witness, The Father is in Me and I in the Father. It is no mere adoption, for He is the true Son of God and cries, He that has seen Me has seen the Father also. Nor did He come into existence in obedience to a command as did created things, for He is the Only-begotten of the One God; and He has life in Himself, even as He that begot Him has life, for He says, As the Father has life in Himself, even so gave He to the Son to have life in Himself. Nor is there a portion of the Father resident in the Son, for the Son bears witness, All things that the Father has are Mine, and again, And all things that are Mine are Yours, and Yours are Mine, and the Apostle testifies, For in Him dwells all the fullness of the Godhead bodily; and by the nature of things a portion cannot possess the whole. He is the perfect Son of the perfect Father, for He Who has all has given all to Him. Yet we must not imagine that the Father did not give, because He still possesses, or that He has lost, because He gave to the Son.
Source: On the Trinity (New Advent)