In a letter to Patricia Warrick (presumably the author of The Cybernetic Imagination in Science Fiction) Dick tries to explain the model of reality he has been constructing over the last seven years.
He claims many of his insights came to him on the edge of sleep during hypnagogic and hypnopompic revelations. One such insight: The macrometasomakosmos (which he is thankfully abbreviating now as MMSK) is a binary signaling system. It micro-adjusts reality by trying out moves and making corrections. Even though these switches happen in a trillionth of a second Dick saw it as a flickering, since during the off state reality ceases to exist. The universe is destroyed and recreated every day. We are left with the best of all possible worlds, although this decision making takes place in the black box of the void outside of our awareness.
He attempts, somewhat shakily, to reconcile Christianity (and Taoism and Gnosticism) with this quantum mechanical computer view of reality to create a new conceptual model. I believe he is saying that Christ enters our consciousness and allows us to comprehend and participate in this binary forking decision tree.
“For the first time in human history” we are perceiving and understanding the world in a new way. He connects what he saw to the double-slit experiment in quantum mechanics and decides that it demonstrates the binary fork decision making, but he has trouble fully describing this model where the Christ consciousness combines the observer and reality field into one.