First of three posts on the grace of Christ
In the Third Part, Question 2, Articles 10, 11, and 12 of the Summa Theologica St. Thomas asks whether the union of Incarnation, the hypostatic union of a human nature with the Divine Nature in the Second Person of the Holy Trinity took place by grace. Theological difficulties say that it did not since grace is an accident, something given freely as a gift or as a grant of friendship, something that happens rather than something substantial about the person’s existence.
Could the most intimate union of creature and God take place, then, by grace? St. Thomas answers yes, but an understanding of the grace of union requires first an understanding of what grace is. Grace suggests two ideas, something given to someone who has found favor with the bestower, and something given as a gift regardless of having found dearness. Thus grace is not a matter of right or something owed.
What God gives to man can never be something acquired by merit, and these gifts are given more abundantly than man deserves. These gifts, once given as grace and received by man, make him more pleasing to God reflecting a more intense divine likeness and stronger unity to Him. The goal of the human life is to be united with God, in knowledge and love for eternity, and to share according to his nature, as much as possible for a creature, in the Divine Nature. Union with God is what makes someone holy because God is the essence of holiness. The closer one is to God, the holier he is.
Christ is both man and God united in the Second Person of the Holy Trinity; therefore Christ’s human nature can be said to be substantially holy because of the hypostatic union. This union is the most intimate union between a creature and God. This special grace, unique only to the Son of God, is called the grace of union, a real fullness of grace. Both given freely as a gift and given in desire to be united, this grace is natural to Christ. He had this grace from His nativity, the beginning of His conception when the human nature was united with the Divine Nature and His soul was filled with the gift of grace.
“And therefore also the Holy which shall be born of thee shall be called the Son of God.” Luke 1:35
Father John A. Hardon, S.J. Christology of Thomas Aquinas. The Real Presence, Section “Grace and Wisdom.”
Fr. Kenneth Baker, S.J. Fundamentals of Catholicism, Volume 2: God, Trinity, Creation, Christ, Mary. (San Francisco, CA: Ignatius Press, 1995), page 260.