In Church Dogmatics II.1 §28, Barth warns against the reduction of the doctrine of God into a form of metaphysical musings about the being of God, which he suggests Melancthon to have done at some length in his Loci of 1521. Barth, here, cautions against simply having a philosophical knowledge of God because it abstracts God from His actions in Christ. It is by God's actions that God is known--He is not bound by them, but they are bound to Him. It is clear for Barth that our quest is not to ask questions about "being," but about God, and "that our subject is God and not being, or being only as the being of God." But, what then is the place of ontology and metaphysical language in Christian theology? Barth himself says, "Yet we must not yield to a revulsion against the idea of being as such..." It seems as though Barth is getting at a post-doctrine of God ontology. That is, once the statement "God is," is addressed, it is then appropriate to engage in a further investigation of "being" as such. But this investigation must always be modified and directed by the statement and presupposition that, "God is."
It is in my own thoughts and studies that I find metaphysical knowledge and language helpful. But it is important that we heed the advice Karl Barth, and resist the temptation to absorb God into another metaphysical category, which leaves discussions of God quite empty. Metaphysical language and musings may be helpful--and often beneficial--though, the theologian must always be reminded that questions of "being" tell us nothing about God, but the statement "God is," tells us about God's being. That God simply is, must be our starting point.
Karl Barth, Church Dogmatics II.1: The Doctrine of God §28, ed. G.W. Bromiley and T.F. Torrance, trans. G.W. Bromiley (London: T&T Clark, 2009), 4
Karl Barth, Church Dogmatics II.1, 4