One of the key points about Barth's Christological anthropology, as I understand it, is that the Christ who stands at the centre of it is the real Jesus of Nazareth, as witnessed in the Scriptures. Anthropology is not shaped around a centre defined by a logical or aesthetically appealing idea, whether that of the ideal human or that of incarnation. It is based around a person, and when we say a person we mean an event - a history, a happening. General anthropology is the periphery to which the life, death, and resurrection of Jesus Christ form the centre.
One of the huge implications of this is that it is impossible that human beings should fail to be human.
Everything we know about the world and the people who inhabit it militates against this conclusion. In fact, we have already seen that the phenomena of human life exhibited all around us speak just as strongly of inhumanity as humanity. If we constructed our anthropology on the basis of experience, we would probably say that people could choose: they could affirm or deny their own humanity. In one sense we would be right. We can affirm or deny our humanity. I can choose to live as someone who stands before God and my fellow man, or I can try not to. I can seek to escape God, either by atheism or religion, and my fellow man, either by solitude or shallow relationships. It is the great tragedy of sin, from the human point of view, that it tends towards the dehumanising of human beings. It attempts to mask what and who we are.
But the event of Jesus Christ is and means 'God with us'. And it means 'he will save his people from their sins'. The presence of Jesus Christ - and especially the resurrection - means that humanity is not abandoned, just as Jesus is not abandoned to corruption. God affirms his creation, and upholds it against its own wilful fall and senseless drive to be nothing.
There is no single human being who can erase what Jesus Christ has done. There is therefore no single human being who can be really without God, or really without the fellow man, because Jesus Christ is both. All my attempted inhumanity cannot unmake me. The mask of evil which I choose to wear - and in so far as it lies with me, I will this mask to be my reality - is in fact not chosen for me, because Jesus Christ is with me as a human being and as my God, and I am upheld.
But how much worse this makes it! Every sin is against my own humanity, against the grace of God, against Jesus Christ. And every sin is an impossible attempt to be nothing when God has made me something. My sin may take me to the fire of hell, but it will not make me less than a person who stands before and with God and my fellow man. And that is a terrifying thought.