38 Put in the dispensation of the flesh which He assumed, and through the obedience whereby He emptied Himself of the form of God, Christ, born man, took to Himself a new nature, not by loss of virtue or nature but by change of fashion. He emptied Himself of the form of God and took the form of a servant, when He was born. But the Father's nature, with which He was in natural unity, was not affected by this assumption of flesh; while Christ, though abiding in the virtue of His nature, yet in respect of the humanity assumed in this temporal change, lost together with the form of God the unity with the divine nature also.
But the Incarnation is summed up in this, that the whole Son, that is, His manhood as well as His divinity, was permitted by the Father's gracious favour to continue in the unity of the Father's nature, and retained not only the powers of the divine nature, but also that nature's self. For the object to be gained was that man might become God. But the assumed manhood could not in any wise abide in the unity of God, unless, through unity with God, it attained to unity with the nature of God.
Then, since God the Word was in the nature of God, the Word made flesh would in its turn also be in the nature of God. Thus, if the flesh were united to the glory of the Word, the man Jesus Christ could abide in the glory of God the Father, and the Word made flesh could be restored to the unity of the Father's nature, even as regards His manhood, since the assumed flesh had obtained the glory of the Word. Therefore the Father must reinstate the Word in His unity, that the offspring of His nature might again return to be glorified in Himself: for the unity had been infringed by the new dispensation, and could only be restored perfect as before if the Father glorified with Himself the flesh assumed by the Son.
Source: On the Trinity (New Advent)