Incarnation of God
Mother Mary is full of sweetness because she was rooted from her youth in obedience, charity, and divine understanding. It is sustained by her nobleness (she is more eminent than angels' holiness), mercy (she suffered torment for the sake of mercy), loving kindness (she does not refuse anyone who reasonably prays for his own good), beauty (the beauty of Mary's soul is above all that is created and the nobleness of her body surpasses all created beings), and divine joy (she has the highest joy in God; nothing pleases her but God). In her, Jesus' Divinity assumed Manhood. Jesus's limbs were like Mary's in shape and appearance, even though Jesus was a man; with no body defects, as there was no sin in Jesus or Mary (Saint Bridget of Sweden, Volume 1, 1360).
Saint Aquinas in 1260 said about the incarnation,
A thing is said to be necessary for a certain end in two ways. First, when the end cannot be without it; as food is necessary for the preservation of human life. Secondly, when the end is attained better and more conveniently, as a horse is necessary for a journey. In the first way it was not necessary that God should become incarnate for the restoration of human nature. For God with His omnipotent power could have restored human nature in many other ways. But in the second way it was necessary that God should become incarnate for the restoration of human nature. Hence Augustine says (De Trin. xii, 10): "We shall also show that other ways were not wanting to God, to Whose power all things are equally subject; but that there was not a more fitting way of healing our misery." Now this may be viewed with respect to our "furtherance in good." First, with regard to faith, which is made more certain by believing God Himself Who speaks; hence Augustine says (De Civ. Dei xi, 2): "In order that man might journey more trustfully toward the truth, the Truth itself, the Son of God, having assumed human nature, established and founded faith." Secondly, with regard to hope, which is thereby greatly strengthened; hence Augustine says (De Trin. xiii): "Nothing was so necessary for raising our hope as to show us how deeply God loved us. And what could afford us a stronger proof of this than that the Son of God should become a partner with us of human nature?" Thirdly, with regard to charity, which is greatly enkindled by this; hence Augustine says (De Catech. Rudib. iv): "What greater cause is there of the Lord's coming than to show God's love for us?" And he afterwards adds: "If we have been slow to love, at least let us hasten to love in return." Fourthly, with regard to well-doing, in which He set us an example; hence Augustine says in a sermon (xxii de Temp.): "Man who might be seen was not to be followed; but God was to be followed, Who could not be seen. And therefore God was made man, that He Who might be seen by man, and Whom man might follow, might be shown to man." Fifthly, with regard to the full participation of the Divinity, which is the true bliss of man and end of human life; and this is bestowed upon us by Christ's humanity; for Augustine says in a sermon (xiii de Temp.): "God was made man, that man might be made God." So also was this useful for our "withdrawal from evil." First, because man is taught by it not to prefer the devil to himself, nor to honor him who is the author of sin; hence Augustine says (De Trin. xiii, 17): "Since human nature is so united to God as to become one person, let not these proud spirits dare to prefer themselves to man, because they have no bodies." Secondly, because we are thereby taught how great is man's dignity, lest we should sully it with sin; hence Augustine says (De Vera Relig. xvi): "God has proved to us how high a place human nature holds amongst creatures, inasmuch as He appeared to men as a true man." And Pope Leo says in a sermon on the Nativity (xxi): "Learn, O Christian, thy worth; and being made a partner of the Divine nature, refuse to return by evil deeds to your former worthlessness." Thirdly, because, "in order to do away with man's presumption, the grace of God is commended in Jesus Christ, though no merits of ours went before," as Augustine says (De Trin. xiii, 17). Fourthly, because "man's pride, which is the greatest stumbling-block to our clinging to God, can be convinced and cured by humility so great," as Augustine says in the same place. Fifthly, in order to free man from the thraldom of sin, which, as Augustine says (De Trin. xiii, 13), "ought to be done in such a way that the devil should be overcome by the justice of the man Jesus Christ," and this was done by Christ satisfying for us. Now a mere man could not have satisfied for the whole human race, and God was not bound to satisfy; hence it behooved Jesus Christ to be both God and man. Hence Pope Leo says in the same sermon: "Weakness is assumed by strength, lowliness by majesty, mortality by eternity, in order that one and the same Mediator of God and men might die in one and rise in the other-for this was our fitting remedy. Unless He was God, He would not have brought a remedy; and unless He was man, He would not have set an example."
Book: Summa Theologica by Saint Thomas Aquinas