I wish to continue preparing for the Octave of Church Unity here by reflecting on Mary. At the third Ecumenical Council, in Ephesus in 431, the Church faced the heresy of Nestorianism. Nestorianism essentially includes the notion that Mary gave birth not to the divine Logos, but only to Christ (or Jesus). Nestorianism threatened to divide Christ into *two* persons (one, the human person Jesus, and the other, the divine Logos), reducing Mary to the mother of only the human person Jesus. The Catholic Church rejected Nestorianism as heretical, because Nestorianism opposes the truth of the incarnation (i.e. that God became man without ceasing to be God), and the incarnation is at the heart of the gospel. The Council taught that Christ was, is, and always will be only *one* divine Person, i.e. the eternal Logos. This fact, however, entailed that Mary is truly the mother of that one divine Person. The bishops, in response to the Nestorian heresy, all agreed that:
"If anyone will not confess that the Emmanuel is very God, and that therefore the Holy Virgin is the Mother of God (Theotokos), inasmuch as in the flesh she bore the Word of God made flesh [as it is written, "The Word was made flesh"]: let him be anathema."
To deny that Mary is the "Mother of God" is equivalent to denying either the deity of Christ, the humanity of Christ or the unity of the person of Christ. And all three of those denials are heresies, because all three of those denials undermine the incarnation, and thus undermine the gospel. The Church taught that because the Second Person of the Trinity is God, and since Mary is the mother of that Second Person of the Trinity, therefore, Mary is rightfully titled the "Mother of God". To deny that Mary is the "Mother of God" is thus to commit one of three possible heresies: (1) a form of Ebionism, i.e. denying that the child conceived within her and birthed by her was actually God, and/or (2) Docetism (or Gnostism), i.e. denying that God actually became human and was physically born, and/or (3) Nestorianism, i.e. denying that Christ is one divine person.
I have been in discussions with some Protestants who deny that Mary is the "Mother of God"; their reason for denying this is because this title is not in Scripture. They claim that calling Mary the "Mother of God" (and especially making it a dogma) is a Catholic addition to the gospel, something forbidden in Scripture (cf. Revelation 22:18), and 'corrupting' our minds "from the simplicity that is in Christ." (2 Corinthians 11:3) And this disagreement divides Catholics from those Protestants who make this claim (which, by the way, is not all Protestants). What is the underlying cause of this disagreement? It is, I think, a result of assuming that Scripture is an exhaustive and fully explicit revelation of the gospel; in other words, it is a result of holding one form of sola scriptura. In this way it fails to realize that the gospel can be attacked and undermined by denying propositions that are not explicitly found in Scripture. It also fails to realize that God gave an enduring magisterium to the Church by which to provide authoritative and binding pronouncements regarding such propositions, and in this way perpetually to preserve and clarify the gospel. (God always seems to use heresy to help the Church grow in her understanding of the gospel.)
There are three other Marian doctrines that are occasions for division between Christians. One is the perpetual virginity of Mary, which was affirmed at the fifth Ecumenical Council (553). Another is her immaculate conception (affirmed in 1854 by Pope Pius IX). And another is her assumption (affirmed in 1950 by Pope Pius XII). Many, if not most Protestants treat these doctrines just as some Protestants treat the title "Mother of God". (See, for example, my recent comments in this thread.) In order to overcome our disagreements about these doctrines, I think we have to step back from the doctrines themselves, if only briefly, and talk about the principled difference between a clarification (or development) on the one hand, and an addition on the other hand. Otherwise we will not know how to distinguish an addition from a genuine clarification. We also have to talk about who has the authority to determine such things. Is the third Ecumenical Council's statement about Mary being the "Mother of God" authoritative because it agrees with our own [or "my own"] interpretation of Scripture, or is it authoritative whether or not it agrees with our own interpretation of Scripture? (That is what I have called The Ecclesial Euthyphro.) If the latter, then on what grounds? Those who deny the existence of an enduring magisterial authority grounded in Apostolic succession through the successive laying on of hands have great difficulty with this question. They typically end up "painting a magisterial target around their interpretive arrow" (something that came up again recently here), rather than face and acknowledge the individualism intrinsic to a position that chooses one's authorities based on their agreement with oneself. (cf. 2 Timothy 4:3)
Since Mary is the Mother of God, and He is our Peace, it follows that Mary is the Mother of our Peace. This is in part why she is called the Queen of Peace, for He is also the Prince of Peace. Grace does not destroy nature, but builds upon it, and elevates it. Mary's biological relation to her Son as Mother (a relation which is also an ontological relation), is not diminished or laid aside, but enhanced and made perpetual by grace, just as the Logos taking on human nature does not diminish His humanity, but elevates it (and all humanity as well!). When Jesus said to John, "Behold, your mother", He was not speaking to John alone, but to all the Church. We are His Body, His brothers, and thus through our sacramental union with Christ she is also our sacramental Mother. (In another way, through our blood relation to Mary we are all blood relatives of Christ, for He received His humanity from her.)
But Jesus also said something to Mary. He said, "Woman, behold, your son." (John 19:26) Again, he was not merely commissioning Mary to care for John; He was commissioning Mary to care for the Mystical Body of her Son, i.e. the Church. She longs for and prays for the full visible unity of His Mystical Body, the Church. When Jesus said, "Saul, Saul, why do you persecute Me?" (Acts 9:4), we see that in a way, the sword still pierces her soul. This too, in a way reveals the thoughts of many hearts. (Luke 2:35) How we relate to Mary can reveal whether we are rightly or wrongly conceiving Christ. Not only that, how we relate to Mary also changes the way we relate to Christ, just as the way we understand Mary reflects and changes how we understand Christ, as we saw above regarding Nestorianism. We cannot take Christ while rejecting His mother, or while belittling her or dishonoring her as though she were no better than ourselves. To love Christ is to love His mother for His sake, and honor her as our Mother. By reflecting on Mary as "Mother of God", and "Queen of Peace", we see that the unity and peace of the Church cannot be something that is achieved apart from her motherly love and compassion. How could the peace of the Body of Christ be achieved apart from the fiat of the Queen of Peace? Her fiat has been elevated into the heavenly realm, and forms the supplication to the Father by which Her Son our Peace, through the Church, is revealed to us, and thereby reveals the thoughts of many, to be either for her Son, or to be in opposition to Him (Matthew 12:30; Luke 11:23), for it is through the Church that the secrets of our hearts are laid bare (1 Corinthians 14:24). To achieve full visible unity, not only do we need to be united doctrinally about Mary, we need to be united in loving devotion to Mary.
Pray for us, Mary our mother, that our sad divisions may cease, and that we may be truly one, for the glory of your Son.