The reason why Eckhart often reads like a heretic is that he was concerned to express not our own knowledge as individual creatures, but the divine knowledge itself, basing himself in the “ground” of the soul. As he says, “only in the ground of the soul is God known as he is,” for there “the intellect knows as it were within the Trinity and without otherness”. This possibility of knowing God “as God knows God” is opened to man by the Hypostatic Union of divine and human natures in Christ. C.F. Kelley’s Meister Eckhart on Divine Knowledge (Yale University Press, 1977) is a masterly study of exactly this point. As Kelley explains:
“A genuine understanding of the principial mode, which is constituted as it were within Godhead, is an understanding of truth that is beyond the potentiality of human cognition, restricted as that cognition is to individuality. Insight into this truth is a possibility only by way of transcendent act, never by way of potentiality. Yet the revelation of the Word is that transcendent act as assented to by the intellect when moved by the detached will to know.”When the perennialist Alvin Moore Jr reviewed Kelley’s book some years ago in Studies in Comparative Religion, he noted with stern disapproval the fact that Kelley (in Eckhart’s name) “equates the Godhead with the Trinity, sachchidananda with nirguna Brahman.” This goes to the heart of the matter. Kelley refuses, in his interpretation of Eckhart, to follow Schuon’s distinction between Being and Beyond-Being (that is, between the “relatively” Absolute and the “absolute” Absolute, thus relegating the Trinity to a subordinate status), or the related distinction between form and substance in the religions which underpins The Transcendent Unity of Religions.
Kelley’s exposition hinges instead on esse or Being, which he translates as isness, the “act or isness of pure knowledge itself, or Godhead”, the “all-inclusive Reality” (p. 42). He even cites Dionysius in support of Eckhart: “We apply the titles of ‘Trinity’ and ‘Unity’ to that which is beyond all titles, designating under the form of Being that which is beyond Being” (p. 30). Kelley means by all this that the Trinity is not, as Schuon would say, simply the “prefiguration of Manifestation in the Principle”, or the relatively Absolute which stands at the summit of Maya, but is itself the unconditioned Absolute about which “nothing can be said” – save what God authorizes us to say. I have written further about this in "Trinity and Creation: An Eckhartian Perspective".
Also recommended is Cyprian Smith OSB, The Way of Paradox (DLT) and Commentaries on Meister Eckhart Sermons by Sylvester Houedard OSB.