Consider the following passages from two theologians at almost opposite ends of the spectrum.
First, J. Gresham Machen, from his book Christianity and Liberalism:
"How, then, shall God be known; how shall we become so acquainted with Him that personal fellowship may become possible? Some liberal preachers would say that we become acquainted with God only through Jesus. That assertion has an appearance of loyalty to our Lord, but in reality it is highly derogatory to Him. For Jesus Himself plainly recognized the validity of other ways of knowing God, and to reject those other ways is to reject the things that lay at the very center of Jesus’ life. Jesus plainly found God’s hand in nature; the lilies of the field revealed to Him the weaving of God. He found God also in the moral law; the law written in the hearts of men was God’s law, which revealed His righteousness. Finally Jesus plainly found God revealed in the Scriptures. How profound was our Lord’s use of the words of prophets and psalmists! To say that such revelation of God was invalid, or is useless to us today, is to do despite to things that lay closest to Jesus’ mind and heart.
But, as a matter of fact, when men say that we know God only as He is revealed in Jesus, they are denying all real knowledge of God whatever. For unless there be some idea of God independent of Jesus, the ascription of deity to Jesus has no meaning. To say, “Jesus is God,” is meaningless unless the word “God” has an antecedent meaning attached to it. And the attaching of a meaning to the word “God” is accomplished by the means which have just been mentioned. We are not forgetting the words of Jesus in the Gospel of John, “He that hath seen me hath seen the Father.” But these words do not mean that if a man had never known what the word “God” means, he could come to attach an idea to that word merely by his knowledge of Jesus’ character. On the contrary, the disciples to whom Jesus was speaking had already a very definite conception of God; a knowledge of the one supreme Person was presupposed in all that Jesus said. But the disciples desired not only a knowledge of God but also intimate, personal contact. And that came through their discipleship with Jesus. Jesus revealed, in a wonderfully intimate way, the character of God, but such revelation obtained its true significance only on the basis both of the Old Testament heritage and of Jesus’ own teaching. Rational theism, the knowledge of one Supreme Person, Maker and active Ruler of the world, is at the very root of Christianity."
Second, Katherine Sonderegger, from her Systematic Theology
"Once again we must quietly but firmly state the Christology cannot be the sole measure, ground, and matter of the doctrine of God; there is more, infinitely more to the One, Eternal God."
Now, contrast both of these claims with the claim of Karl Barth in volume IV of Church Dogmatics:
"That God as God is able and willing and ready to condescend, to humble Himself in this way is the mystery of the "deity of Christ" - although frequently it is not recognised in this concreteness. This deity is not the deity of a divine being furnished with all kinds of supreme attributes. The understanding of this decisive christological statement has been made unnecessarily difficult (or easy), and the statement itself ineffective, by overlooking its concrete definition, by omitting to fill out the New Testament concept "deity" in definite connexion with the Old Testament, i.e., in relation to Jesus Christ Himself. The meaning of His deity - the only true deity in the New Testament sense - cannot be gathered from any notion of supreme, absolute, non-worldly being. It can be learned only from what took place in Christ. Otherwise its mystery would be an arbitrary mystery of our own imagining, a false mystery. It would not be the mystery given by the Word and revelation of God in its biblical attestation, the mystery which is alone relevant in Church dogmatics. Who the one true God is, and what He is, i.e., what is His being as God, and therefore His deity, His "divine nature," which is also the divine nature of Jesus Christ if He is very God - all this we have to discover from the fact that as such He is very man and a partaker of human nature, from His becoming man, from His incarnation and from what He has done and suffered in the flesh. For - to put it more pointedly, the mirror in which it can be known (and is known) that He is God, and of the divine nature, is His becoming flesh and His existence in the flesh."
A dissertation's worth of stuff could be said on the relationship between these three passages. And a litany of proof-texts could be given in reply to Machen and Sonderegger. For example,
"No one knows the Son except the Father, and no one knows the Father except the Son and those to whom the Son chooses to reveal him."
"For in Christ all the fullness of the Deity dwells in bodily form..."
One could also make some interesting historical observations in relation to these passages. For instance, to the extent that liberal theology was an attempt to recover the significance of the concrete person of Jesus for Christian theology and life, then Barth can indeed be seen as a child, even an heir, of liberalism. Whatever else it may have been, Barth's break with liberalism in the first decade of the 20th century was not a return to orthodoxy. The standard American description of Barth as "neo-orthodox" is for this reason a complete misnomer. Barth had no interest in reviving orthodoxy as such. Barth's radical Christocentrim - and it is easy to forget how radical it is when you study at the University of Aberdeen (described by one theologian as the home of "radical-apocalyptic Barthianism") - blocked the way for any simple return to orthodoxy as expressed in the mode of classical theism. For Barth, there can be no grounding of Christianity in "rational theism," even one which is based on Scripture. And there is no getting behind Christ to a God who is more rich than the one revealed in Jesus of Nazareth. One can of course contest Barth on these points - though the contest will be most fruitful simply in the doing of one's work, and not in methodological squabbles. And even if one agrees with Barth on the way in which theology must proceed, one can and should contest the conclusions which he draws. If being a Barthian (my supervisor hates this word, as did Barth, who famously said "If there are Barthians, I am not one of them") means anything, it means doing theology the way Barth sought to do theology; it should never mean repristination. Barth claims that "back to" is never a good slogan in theology. This is equally true of any calls to go "back to Barth."