If that didn't twist your brain up a little, these next few chapters will do the trick. Lewis sets out here to discuss one aspect of Theology, the Trinity (the Father, Son, and Holy Spirit) as well as how man can become a part of God, while still remaining man. I'm going to go ahead and admit, this was one of the more difficult chapters for me to understand and to summarize, so if you catch any errors please feel free to let me know. I think Book Four is the most challenging section to process because it deals with such complex ideas, but we'll press on and see how it goes!
Now, notice that each dimension still contains the components of the dimensions before it, so the two-dimensional square is made up of several one-dimensional lines, and the three-dimensional cube consists of both one-dimensional lines and two-dimensional squares. "In other words, as you advance to more real and more complicated levels, you do not leave behind you the things you found on the simpler levels: you still have them, but combined in new ways - in ways you could not imagine if you knew only the simpler levels."
We can apply this principle to the spiritual realm in the idea of the Trinity. The human level is the simpler dimension where each person is one individual being, and two people are two distinct, separate beings. Moving up to the level of the Divine, though, we find there are still distinct personalities, but they are combined in ways we cannot fully comprehend with our human minds. "In God's dimension, so to speak, you find a being who is three Persons while remaining one Being, just as a cube is six squares while remaining one cube." But because this dimension is beyond the dimension we know and experience as humans, we aren't completely capable of making sense of or even imagining this type of existence.
So why even bother talking about something we can't understand or comprehend?
"Well, there isn't any good talking about Him. The thing that matters is being actually drawn into that three-personal life, and that may begin any time... tonight, if you like."
"If Christianity was something we were making up, of course we could make it easier. But it is not. We cannot compete, in simplicity, with people who are inventing religions. How could we? We are dealing with Fact. Of course anyone can be simple if he has no facts to bother about."