Classical theism has said that we can know that God is but not what God is. We cannot know God’s essence. They had a reason for saying this: in the Hellenistic world (which was never fully rejected in Christendom and received a shot in the arm in Descartes, even until our present day) to know something was to “master” it and bring it under one’s control--it is to subsume all under “The Same” (Smith 31).
Thus, when the Fathers said we cannot know God’s essence (cf. Basil, Letters 231 and 234) they meant we cannot bring it under our grasp, seizing and dominating it. And this is virtually the position of Christendom, East or West. Later Western thought nuanced this approach with the valid distinction between apprehending and comprehending. If the field of debate is Hellenism’s view of knowledge as domination, then I fully agree with the tradition on this point.
But why should we let idolaters determine how we can talk about God? I understand the distinction between knowing what/that God is, and I agree with the value it seeks to protect, but with Barth I think it is simply “impracticable” (II.1/187). Jesus said if we know him, we know the Father. Presumably we know the person of the Father. Can we know the person of the Father without knowing what the Father is? This is where classical metaphysics comes undone. No one is saying we can “fully exhaust God’s essence.” But neither do I think Jesus is saying, “You can know the Father but I am holding back on you. You can only have me in reserve.”
We can’t separate person from nature (assuming classical metaphysics for a moment). If we know Jesus, and if Jesus is homousios with the Father, then does it really make sense to speak of a free-floating, abstract essence behind the two? If you still maintain that the above classical metaphysics is a coherent model, then the only thing left is the frozen theology of Palamism and Eastern Orthodoxy. They at least are consistent. They posit that the persons and the nature is hyperousia, beyond being (Palamas, Triads 2.iii.8; 3.iii.17-20). Still, I must cut this option off at the pass. I echo Robert Jenson: if the Persons are eclipsed by the energies and remain in the realm of hyperousia and “above” the biblical narrative, in such case that we can no longer identity the persons by their hypostatic propria, we can only conclude that Palamism, despite its best intentions, is a more frozen form of modalism than anything Augustine or Aquinas ever dreamed of.
Barth, Karl. Church Dogmatics.
Palamas, Gregory. The Triads, ed. John Meyendorff.
Smith, James K. A., jacques derrida: live theory.