The Nativity of Christ, or Christmas ("Christ Mass"), celebrates one of the most important events of the Church: the incarnation of the Son of God. Like the Trinity, the Virgin Birth, the Resurrection, etc., the Incarnation is one of those rock-bottom Christian beliefs that most Christians assent to but probably don't really understand. Though Catholics all over the world affirm their belief in the incarnation every Sunday by reciting the Creed, how many could explain this tenet of the faith in the simplest terms?
Let's start with a story. . .
The archangel Gabriel appears to Mary and announces to her that God has chosen her to be the mother of the Christ Child, His Son. Mary says, "Your will be done" and the Holy Spirit descends on Mary, giving her the child. Nine months later the Christ is born in Bethlehem.
Simple enough story, right? If we left the incarnation there, we would still have the basic truth of Christ's arrival into the world. Things get a little more complicated when we start to think about what it means for the Son of God (who is God) to take on human flesh and live among us. How does the God of the Old and New Testament become incarnated yet remain sovereign God? We are immediately confronted by what theologians call "the Christological question": how is the man Jesus also God?
Before this question was settled by the Council of Nicaea in 325 A.D., a number of answers were offered and rejected:
Jesus is really a man who possesses God-like qualities.
Jesus is really God in the appearance of a man.
Jesus is half-God and half-man.
Jesus' soul is divine but his body is human.
Jesus' body is human but his mind is divine.
Complicating matters even more was the lack of an adequate theological vocabulary with which to think about and write about the incarnation. Early Christian theologians turned to the available philosophical vocabularies for help. The most prominent philosophical system in the first few centuries of the Church was a developed form of Platonism. Borrowing heavily from the Platonists, the Church Fathers crafted a creedal statement that said: The Father and the Son are the same in substance ("consubstantial"), meaning that they are the same God.: "God from God, light from light, true God from true God." The Son was not created in time like man but rather begotten from all eternity. He "became incarnate" through the Virgin Mary--fully human in all but sin.
This creedal statement defined the orthodox position of the Catholic Church. However, interpretations of the creed abounded and additional councils had to sort through them all in order to discover the orthodox expression of the true faith. In the end, the Nicene Creed was taken to mean that Jesus was fully human and fully divine: one person (one body/soul) with two natures (human and divine). "Person," "essence," "being," "nature" are all terms borrowed from Greek philosophy. So, as the West discovered new ways of thinking philosophically, these terms took on different meanings and our interpretations of theological expressions of the truth developed as well. The basic truth of the incarnation does not change; however, how we understand that truth does change.
For example, the Greek word we translate as "person" is prosopon, or mask. This term was used in the Greek theater to denote the different characters played by one actor. A single actor would hold a mask in each hand and shift the masks in front of his face to say his lines, indicating that the lines were being said by different characters. Applying this term to God, the Blessed Trinity, we arrive at a single actor (God) using three masks (Father, Son, Holy Spirit). Same actor, different characters. Ultimately, this metaphor is woefully inadequate for expressing the deepest truth of the Trinity. Yet, we still say that the Trinity is three divine persons, one God. "Person" as a philosophical term used to describe a theological truth had to be developed.
Eventually, we came to understand several vital distinctions: The Church uses the term "substance" (rendered also at times by "essence" or "nature") to designate the divine being in its unity, the term "person" or "hypostasis" to designate the Father, Son and Holy Spirit in the real distinction among them, and the term "relation" to designate the fact that their distinction lies in the relationship of each to the others (CCC 252).
So, God is one substance; three divine persons; distinguished from one another not by their natures or persons but by their relations one to another. The incarnation then is the second divine Person of the one God becoming a human person with two substances or natures.
You are one person with one nature: "I am human."
God is three divine persons with one nature: "I am divine."
Christ is one person with two natures: "I am human and divine."
Aquinas, quoting Irenaeus, writes, "God became man so that man might become God." The incarnation of the Son makes it possible for us to become God (theosis). This is how Catholics understand salvation.