51 He who has not grasped the manifest truths of the faith, obviously cannot have an understanding of its mysteries; because he has not the doctrine of the Gospel he is an alien to the hope of the Gospel. We must confess the Father to be in the Son and the Son in the Father, by unity of nature, by might of power, as equal in honour as Begetter and Begotten. But, perhaps you say, the witness of our Lord Himself is contrary to this declaration, for He says, The Father is greater than I. Is this, heretic, the weapon of your profanity?
Are these the arms of your frenzy? Has it escaped you, that the Church does not admit two Unbegotten, or confess two Fathers? Have you forgotten the Incarnation of the Mediator, with the birth, the cradle, the childhood, the passion, the cross and the death belonging to it? When you were born again, did you not confess the Son of God, born of Mary? If the Son of God, of Whom these things are true, says, The Father is greater than I, can you be ignorant that the Incarnation for your salvation was an emptying of the form of God, and that the Father, unaffected by this assumption of human conditions, abode in the blessed eternity of His own incorrupt nature without taking our flesh?
We confess that the Only-begotten God, while He abode in the form of God, abode in the nature of God, but we do not at once reabsorb into the substance of the divine unity His unity bearing the form of a servant. Nor do we teach that the Father is in the Son, as if He entered into Him bodily; but that the nature which was begotten by the Father of the same kind as His own, possessed by nature the nature which begot it: and that this nature, abiding in the form of the nature which begot it, took the form of human nature and weakness.
Christ possessed all that was proper to His nature: but the form of God had departed from Him, for by emptying Himself of it, He had taken the form of a servant. The divine nature had not ceased to be, but still abiding in Him, it had taken upon itself the humility of earthly birth, and was exercising its proper power in the fashion of the humility it assumed. So God, born of God, being found as man in the form of a servant, but acting as God in His miracles, was at once God as His deeds proved, and yet man, for He was found in the fashion of man.
Source: On the Trinity (New Advent)