For Barth, Christology, who Christ is, is at the heart of all of dogmatics. To fail to answer the question of Christology is to fail in all of theology. Christ and who he is lies at the center of Barth's entire undertaking. And, at the heart of Christology is the problem of the incarnation, or how the Word becomes flesh and unites the divine and human nature. This problem of Christology as Barth calls it must remain a mystery. There is no explaining this mystery away, but one must let it persist as a mystery.
Barth is careful (as always) to protect the freedom of God in the incarnation. That is, there was no necessity either inside or outside of God that compelled the incarnation. It was not the necessary consequence of creation or even the fall. The incarnation remains God's free act of mercy. As Barth writes:
"That is, in His Word becoming flesh, God acts with inward freedom and not in fulfillment of a law to which he is supposedly subject." (CD I.2 §15.2 p. 135).Further protecting God's freedom is Barth's conclusion that the incarnation in no way limits or diminishes God in any way. The Word of God, the λόγος, remains the Word of God in all of His deity.
Yet, the Word of God really does take on flesh. He really assumes human nature "like us." Barth is clear that even though the divinity of God is not diminished through the incarnation, still, the incarnation is a true assumption of human nature "like us." Jesus is not a demi-god or an ideal man, he is a man "like us." For Barth, if in the incarnation God did not become "like us" he would be of no help to us.
Barth is careful to avoid any form of adoptionism. There was not first a man Jesus whom the Word of God assumed. No, This was a literal incarnation, the human nature united with the divine nature in the person of Jesus Christ.
For Barth, σάρξ, is a neutral or negative term. In its neutral sense, it is merely the stuff we are made of. In the negative sense, is it what separates humans from God and other animals. It is the result of the fall and what makes humans liable to the judgment of God. In the incarnation, Christ assumes flesh in both senses, but more importantly in the second sense. In this sense, Christ takes on human flesh in all of its liability to the judgment of God. This is the mystery and the miracle of the incarnation. That God takes on the very nature of man that is liable to judgment is inconceivable. As Barth writes:
"He would not be revelation if He were not man. And he would not be man if he were not 'flesh' in this definite sense. That the Word became 'flesh' in this definite sense, this consummation of God's condescension, this inconceivability which is greater than the inconceivability of the diving majesty and the inconceivability of human darkness put together: this is the revelation of the Word of God." (CD I.2 §15.2 p. 152).Indeed for Barth, the greatness of Christ, his perfection, lay not in his good deeds or special morality, but in his obedience, taking on human flesh in all of its liability to God's judgment, and doing so willingly.
And here is the money quote:
"Mariology is an excrescence, i.e., a diseased construct of theological thought. Excrescences must be excised." (CD I.2 §15.2 p. 139).If you had any doubt about how Barth feels about the Catholic practice of the veneration of Mary, take a look at the definition of the word "excrescence" from dictionary.com:
1. an abnormal outgrowth, usually harmless, on an animal or vegetable body.2. a normal outgrowth, as hair or horns.3. any disfiguring addition.