In Christ, God and man have exchanged the ultimate gift: both live together now in one Person. (Reason 4 for the Incarnation: That we might partake of the divine nature.)
Here is Blessed Dom Marmion on this ‘admirable exchange’ of gifts, the divine nature and the human nature exchanging themselves with each other on Christmas morning:
In us likewise there will henceforth be two lives. The one, natural, which we have by our birth according to the flesh… The other life, supernatural… It is this life that God communicates to us by His grace, since the Incarnate Word merited it for us.
God begets us to this life by His Word and the infusion of His Spirit, in the baptismal font… It is a new life that is superadded to our natural life, surpassing and crowning it… It makes us children of God, brothers and sisters of Jesus Christ, worthy of one day partaking of His beatitude and glory.Of these two lives, in us as in Christ, it is the divine that ought to dominate–although in the Child Christ the divine life is not as yet manifested, and in us it remains ever veiled under the outward appearance of our ordinary existence.
It is the divine life of grace that ought to rule and govern, and make agreeable to our Lord all our natural activity thus deified in its root.
Oh! if the contemplation of the Birth of Jesus and participation in this mystery by the reception of the Bread of Life would bring us to free ourselves, once and for all, from everything that destroys and lessens the divine life within us; from sin, wherefrom Christ comes to deliver us… from all infidelity and all attachment to creatures; from the irregulated care for passing things… from the trifling preoccupations of our vain self love!
If we could thus be brought to give ourselves entirely to God, according to the promises of our baptism, when we were born to the divine life; to yield ourselves up to the accomplishment of His will and good pleasure, as did the Incarnate Word in entering into this world; to abound in those good works which make us pleasing to God:
Then the divine life brought to us by Jesus would meet with no more obstacles and would freely expand for the glory of our Heavenly Father; then we who are bathed in the new light of the Incarnate Word should show forth in our deeds what by faith shines in our minds; then our offerings would befit the mysteries of the Nativity.
[I have taken the liberty of rendering what, to me, are the most sublime turns of phrase in bold.]