In his quiet way, Martin Buber might be the most important figure in 20th-century philosophy and theology. If existentialism killed the louse of logical positivism (and I believe it did for the most part, although positivism still exists to some extent in scientific communities), and if existentialism sowed the seeds for structuralism and poststructuralism (and that case can be made, even if I'd be embarrassed to call myself an existentialist after you made it), then credit must be given where it is due, and Buber should get a large bronze statue in Paris or wherever else you want to put the capital of philosophy.
Buber wrote his classic I and Thou in 1923, four years before his fellow German Martin Heidegger coughed out his nearly comprehensible but nevertheless brilliant Being and Time, which takes a good deal of theory about Being and Existence (and curiosity, and angst!) from Buber's book. From Heidegger sprung Sartre, and from Sartre sprung...well, basically everyone else. And so Buber is as important as Kierkegaard and Nietzsche for the development of Existentialism--and he's more important than Edmund Husserl. And yet to some extent he stands on the outskirts of the movement.
Perhaps it's because his concern is not an abstract notion of Being but with Ultimate Being, or God. In this, it is true, he has more in common with the Christian existential theologians--Karl Barth and Paul Tillich and a host of less important figures. Indeed, it is sometimes hinted (as it was in a class I took at my religious college, in which we received a very brief and simplified version of the I and the Thou) that Buber is some kind of closet Christian--a Jew, certainly, but one who was a Christian at his core.
Walter Kaufman bemoans such a viewpoint in his wonderfully poetic introduction to the book. For one thing, he points out, I and Thou is fundamentally anti-theological, leveling all notions of God and starting from scratch, and if that scratch references the New Testament, it spends just as much time in Eastern texts and in daily life. For another--and it's here Kaufman and I part philosophical ways--Christianity based more on Greek culture than on Judaism, and as Buber was profoundly Jewish (he repopularized Hasidism, for example), he could not have been a closet Christian. And in fact, he viewed Christianity as an objectification of God and the Eucharist as a symbol of that objectification. And so Kaufman is baffled that Christian theologians have so latched on to I and Thou.
The reason for that, I suspect, is similar to the reason that the early Church latched on to the Hebrew Bible. The early Church, of course, was composed to a very large extent of cultural and religious Jews, and for the most part the existentialist theologians lack this background in Jewish culture--but their attitudes toward the Old Testament and toward Buber are similar in that the Christian groups saw their philosophy as a fulfillment of the older one.
I and Thou is a wonderful book in many ways. It's mystic yet grounded in reality, and its author's prose is more poetic and beautiful than any other 20th-century existentialist. (The century qualification is important, since Kierkegaard and Nietzsche are both beautiful writers.) And the ideas speak truthfully, as well, even if they are frequently oversimplified.
Here's my oversimplification--and I'll let you find the echo in Heidegger and Sartre and everyone else. We can look at the world one of two ways, either I-It or I-You. The first treats the Other as an object, the second as a subject. The difference between these two is the difference between experience and relation; it is the difference between alienation and wholeness; it is the difference between fantasy and reality. And when we treat any individual as a You, we necessarily treat God as a You.
There is much more to it than this brief paragraph, of course--Buber speaks for more than 150 pages, and each page is important. But one thing he does not provide us with is a guide for entering into the world of the I-You relation, a path out of the world of the I-It experience. Indeed, he tells us, such a guide is self-negating: "Going forth is unteachable in the sense of prescriptions. It can only be indicated--by drawing a circle that excludes everything else. Then the one thing needful becomes visible: the total acceptance of the present." This is a frustrating answer, of course, because we want an answer from Buber. We have sought the sage on the mountaintop, and all he can do is tell us what is wrong rather than giving us a way to fix it.
In fact, we must confront the You as a person, but the You of Buber's God is intangible, even if He is present everywhere. God reveals Himself to us directly, he says; He reveals Himself to us as a person rather than as a collection of knowledge.
All of this relates so closely to Christ that I cannot believe Buber did not see it. He must have seen it. If God confronts us personally--and if flesh is not evil, as Buber repeatedly claims it is not--then the logical thing would be for God to confront us in the flesh of a person. This is, of course, where the Christian theologians pick up Buber's thread. God confronts us directly in the person of Christ.
Buber disagrees, however. God presents us personally and yet vaguely as the Foundation of All Being, the You behind all other Yous. Remember, if we address any particular being as You, we are addressing Being Itself, i.e., God, as You. So we end up with a paradox, with a God who addresses us directly but indirectly, as a person but through all things.
The Christian theologians, I suspect, would say that we're too stupid or too fallen to deal with this paradox. Karl Barth, for example, would tell us that our capacity for making Towers of Babel out of religious figures means that faced with this system, we will take the I-You world and adapt it into idolatry. We will take something meant to end our alienation--relationship--and run it into the ground.
Buber hints at the solution; indeed, he nearly spells it out: "Whoever would settle the conflict between antinomies by some means short of his own life transgresses against the sense of the situation." The Christian posits, then, that Christ's death on the cross settled the conflict between antimonies with His own life, thus providing a specific end to alienation that nevertheless results in a redemption of the world itself, in the form of the doctrine of the Incarnation.
Christian existentialists, then, would see their work as building off of Buber the way the New Testament builds off of the Old, clarifying and satisfying it. (Such an attitude, I understand, is offensive to the Jewish faith, and I do not mean for it to be so. But there's no way around the Christian assertion that Christianity fulfills and thus outdoes Judaism.) But the difference is here: the Christian Bible takes a revelation that is fundamentally limited (God has chosen the Hebrew people for His glory) and expands it (the Gospel is now open to all people everywhere). But Buber's Christian glossers take a universal message (return to the sphere of the I-You is possible for all people everywhere at all times) and make it narrow (that's true--but only through Christ). It's no wonder Walter Kaufman was disgusted.
Has violence been done to Buber in this translation of his ideas into Christian thought? Probably some. But as Kaufman acknowledges in his notes on his linguistic translation of Buber, it's hard to change languages without doing some violence.