There will not be a full review of the final section of C.S. Lewis's classic because Mormons can get the least out of it. As the name "Beyond Personality: Or First Steps In the Doctrine of the Trinity" implies, it is a defense of Trinitarian beliefs. Although he might say one or two phrases that Mormons could recognize, the concepts are completely opposite of each other. It is for this reason that I will only touch on his main themes rather than a longer review. To do otherwise is to not say much good and therefore end up sounding more hostile than intended.
To start with, he should have taken up the advice of those who said, "the ordinary reader does not want theology," because he ends up alienating most people he intends to educate. He still holds Christianity as a supreme religion with flimsy proof. This is especially the case when he states, "We cannot compete, in simplicity, with people who are inventing religions. How could we? We are dealing with fact" (pg. 165). Such a bold statement is hard to believe considering most of his arguments are, by his own admission, philosophic theories. Not to mention there are very few simple religions without a degree of stereotyping creating that impression. Some religions are numerically larger and existed longer than the upstart Christianity he is so fond of defending.
Most of his theory in this section breaks down into two parts. He calls the first Bios, or the physical and the second Zoe or the eternal spiritual. Like Greek philosophy the physical is seen as simply a false "statue" of the super-reality living spiritual world. He breaks the difference between humans and Jesus Christ as "making" and "Begetting". Something is made that is not oneself and another thing is begotten that is born near identical. He completely rejects the Mormon teaching that humans had a spirit born of God before mortality even as there was a physical creation. To C.S. Lewis, humans are compared to slugs or crabs.
As was said, his main defense is the doctrine of what he calls "The Three-Personal God" of Trinitarians. To help his readers understand the concept he compares it to a Two Dimensional person trying to understand a Three Dimensional world. From a limited perspective a cube looks like a square (pg. 162). The Trinity is compared to a group of people that form a corporate behavior, yet rejects any individuality in the relationship. Jesus existed eternally with God so that God could have someone to Love (a very important attribute to His nature), and yet Jesus is part of God. It ends up sounding like a shallow self Love of a split personality. He concludes his description of the Trinity by saying God could be considered in front of you noticed, Jesus as beside you helping, and the Holy Ghost within or behind you. Although he talks of them as super-personalities, they are far from actual people. For Christians, compared to all other religions, "God is not a static thing - not even a person - but a dynamic, pulsating activity, a life, almost a kind of drama" (pg. 175). It is that old oxymoron of God as more real that reality, but yet more of a concept than a thing. It is at this point that a full review becomes difficult. If Mormonism is compared to "Science-Fiction" than it is only right to compare most other religions as "Fantasy" with the convoluted explanations.
Once again C.S. Lewis rejects an idea from other religions that he co-ops in a different disguise for his Christian beliefs. He says:
Again, some people think that after this life, or perhaps after several lives, human souls will be "absorbed" into God. But, when they try to explain what they mean, they seem to be thinking of our being absorbed into God as one material thing is absorbed into another. They say it is like a drop of water slipping into the sea. But of course that is the end of the drop. If that happens to us, then being absorbed is the same as ceasing to exist . . .
With some irony that he doesn't acknowledge, he states, "It is only the Christians who have any idea of how human souls can be taken into the life of God and yet remain themselves - in fact, be very much more themselves than they were before" (pg. 161). He doesn't reject the concept, but only the details. Here is where his belief in Trinitarianism is coupled with the rest of what he has said before. He had hinted that the Atonement brings humans to a higher spiritual level, and now he will say what is important about that level. At first his idea that "if we share in this kind of life we also shall be sons of God" and "Every Christian is to become a little Christ" (pg. 171) could be acceptable to Mormons. However, his meaning is very different. As more than implied above, the reason for Christ's atonement is to help humans be "absorbed" into God. In effect, ultimately those who are saved will become more than like God, but become a part of God. The logical conclusion that it is hard to know if he recognizes is that "The Three-Personal God" with adding humans becomes "The Billion-Personal God" once salvation is reached. How this works with his belief in the Resurrection of the body is never explained. There are just too many holes that will be put at arms length.
Probably the most helpful chapter for Mormons is the one that describes God as outside of time (pg. 166-171). The best way to describe it is that Time is always in front of God, with Him living in the eternal Now. His theories are similar to what some Mormons have speculated on the subject. It is worth reading if all other chapters are skipped.
Reading all the other sections of "Mere Christianity" gave some new perspectives that enhanced Mormon religious views, even with objections. The final section was frustrating. It seemed to undo all the good that had been done with the rest of the book. With a large grain of salt, it might cause someone who is looking at Christianity seriously for the first time to be persuaded to become atheist. Like it or not, C.S. Lewis seemed to be preaching to the Choir. The rest were at times denegrated.