Read about it here: http://www.firstthings.com/article/2014/06/karl-barths-failure
‘Modern philosophy assumes the falsity of classical theism. It begins by discarding, not disproving, the family of arguments that provide the metaphysical grammar of Christian orthodoxy. Barth followed suit—and the results were fatal.
Barth yielded to modernity’s most pernicious idea, which took aim not at belief in the supernatural but at our rational capacity for knowledge of it. In denying what Jesuit philosopher Bernard Lonergan called the “native infinity” of human understanding, Barth capitulated where he most needed to take a stand. He seemingly did not understand that restricting reason was modern philosophy’s great act of presumption, not humility. Nor did he understand that rejecting the secularity of reason was Christian philosophy’s great act of piety, not hubris. And his bargain with Kant—turning the limits of reason into an opening for revelation—could only corrode the foundations of Christian faith.
By rejecting the speculative power of the intellect, Barth was drawn into making two mistakes. First, he turned his back on the metaphysics of classical theology, rendering almost unintelligible the conceptual idiom of the doctors and creeds of the Church. Barth did not hide this, and he worked hard to square his dogmatics with Christian tradition, replacing appeals to nature and causality with appeals to history and narrative, but the result was that he could not properly and consistently distinguish God’s nature from his actions in the history of salvation.
Barth’s second and deeper mistake was to sever the mind’s speculative relation to God. He dissolved the classical synthesis of faith and reason, collapsing all theological understanding into an exercise of faith. Unable to appeal to truth besides Jesus Christ, Barth was powerless to explain how truth could be known and communicated without supernatural assistance. He was even pressed to invoke divine revelation as proof of the existence of the external world, a sign something had gone very wrong.
His basic error is evident in his rejection of natural theology, which holds that careful observation of contingent beings can disclose the necessary being of God. This argument comes in several permutations, most of which are sketched by Thomas Aquinas, but its success in demonstrating God’s existence was arguably a secondary concern. The primary purpose of traditional natural theology was to show the indissoluble connection between the human intellect and a transcendent God who is Being itself.
Barth’s charge that some natural theologies compromised divine transcendence was true enough, but his indictment was indiscriminate. He did not appreciate that classical natural theology aimed at clarifying the proper reach and function of natural reason: that we can know with certainty that God exists but cannot understand his divine essence in itself. This teaches us both the nobility of reason (knowing that God is) and its radical insufficiency (not knowing what God is).
He simply could not allow that a genuinely philosophical understanding of God is demanded by the intellect’s desire to know. He wanted to sharpen his dispute with classical theism so as to make it entirely about the revealed nature of God. But this could not succeed, if only because what one holds about God is informed by a host of philosophical commitments. For its part, classical theism maintained that Christian belief both presupposes and propels philosophical inquiry. It acknowledged, even celebrated, that Christian belief is committed to philosophical positions concerning the intelligibility of the natural world, the power of the human intellect to understand that world, and our capacity to communicate truth. (Hence the First Vatican Council’s condemnation of those who denied that God can be known with certitude by the natural light of human reason.)’