Yesterday I was visiting with my mother and she asked what I mean when I write about Jesus making a difference for human history. I've been thinking about it. This is an important question, because it goes to the heart of why Christianity should be worth anyone's time or effort, or why we call it the good news.
We have to start with a properly metaphysical reflection. What makes mutal relationship possible between two beings? How is it that two people are able to communicate, to trust each other, to have what we call a personal relationship? They are able to relate, to communicate because precisely because they share something in common, and that's what we call human nature. This is why the same depth of relationship is not possible with a cat, because human nature and cat nature are not quite the same thing. But it also means that we are able to relate with a cat more than we can with a bat. And we can empathize with the bat better than we can know what it's like to be a tree or a rock. Human nature and tree or rock nature are just so far apart that we can't even guess what it means to have a personal relationship with these creatures.
It's the shared nature that makes relationship and communion possible. So what of us and God? Certainly human nature and divine nature are infinitely separate, by virtue of God's Infinity. It would seem that there could be no relationship between God and us. Now God can communicate in a mediated way, as he did when he appeared in the fire and storm with Moses, or in the "still small voice" of the prophet as he revealed to Elijah. But what of direct mutuality with God; is such a thing possible?
The good news of Christianity is that God has closed the gap between human and divine nature in the life of one historical human person, Jesus Christ, by the Eternal Word's unity with this particular human nature. Thus for Jesus Christ the two natures existed as one, but without confusion or division. In him perfect communion between human and divine was established.
The good news for us is that God the Holy Spirit makes Jesus' relationship to God available to each human person and all human nature. By baptism we are given the chance to have the same relationship of communion with God that Jesus had. The Holy Spirit is precisely this availability of the great event of the Incarnation.
This is why the great celebration of Christianity is precisely holy communion, in which the "sacred exchange" between human and divine nature is exercised and made present.