Leave it to Barth to see an analogy of the incarnation (the hypostatic union) as the inner-theological basis of the Great Commandment found in Matthew 22:37–40. Let me share that now, with a concluding remark following. It is taken from Barth’s Church Dogmatics III/2 §45:
For a true understanding, we can and must think of what is popularly called the twofold law of love—for God and neighbour (Mk. 12.29-31 and par.). It is no accident that it was Jesus who summed up the Law and the prophets in this particular way. He was speaking primarily and decisively of the law of His own twofold yet not opposed but harmonious orientation. He declared Himself, and therefore the grace of God manifested in Him, to be the sum of the Law. The two commandments do not stand in absolute confrontation. It is clear that Jesus did not regard love for God and love for the neighbour as separate but conjoined. Yet they are not identical. In Mt. 22.38 the command to love God is expressly called the first and great commandment, and the command to love the neighbour is placed alongside it as the second. God is not the neighbour, nor the neighbour God. Hence love for God cannot be simply and directly love for the neighbour. Yet the command to love the neighbour is not merely an appended, subordinate and derivative command. If it is the second, it is also described as like unto the first in Mt. 22.39. A true exposition can only speak of a genuinely twofold i.e., a distinct but connected sphere and sense of the one love required of man. It has reference to God, but also to the neighbour. It has the one dimension, but also the other. It finds in the Creator the One who points it to this creature, fellowman. And it finds in this creature, fellow-man, the one who points it to the Creator. Receiving and taking seriously both these references in their different ways, it is both love for God and love for the neighbour. Thus the structure of the humanity of Jesus Himself is revealed in this twofold command. It repeats the unity of His divinity and humanity as this is achieved without admixture or change, and yet also without separation or limitation.[1]
What a beautiful explication of our passage (Mt. 22:37-40): “Jesus said unto him, Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind. This is the first and great commandment. And the second is like unto it, Thou shalt love thy neighbour as thyself. On these two commandments hang all the law and the prophets.” For Barth, the ground of love, is indeed, God’s triune life. But as God has elected freely to come into union with our fallen humanity, thus giving us His elect humanity in Christ (in the great reversal), humanity, in Christ’s vicarious humanity for us, comes to participate in the eternal life of love that God indeed is. It is because of who God has been, and is, as Father, Son, and Holy Spirit, that “He first loved us, that we might love Him [and our neighbors].” And it is within this nexus of miraculous grace, of God become human, that the impossible possibility of us coming to have capacity to love God, and then others, as the necessary and organic consequent of that, comes to fruition.
Some unitarians might want to argue that the grammar and thinking of Nicaea and Chalcedon goes beyond Holy Scripture. But in fact, as Barth so lucidly recognizes, it is that very reality, which has come to be signified by said grammar, that underwrites, not only the Dominical teaching, but the total teaching of both the New and Old Testaments. The very sum of the whole old covenant, as Barth rightly recognizes in Jesus’ framing, proleptically anticipates the fact that the Son of Man would indeed finally come bringing God and humanity, humanity and God, God and neighbor, neighbor and God into an elevated participatory life with Him; sharing in the glory that the Son has always already shared with the Father by the Holy Spirit, indeed as the Deus incarnandus (‘the God to be incarnate’).
[1] Karl Barth, Church Dogmatics. The Doctrine of God III/2 §45 (London/New York: T&T Clark, 2010), 14.