C. S. Lewis defends Christianity by building a logical foundation for belief and constructing an entire theology upon that foundation. He begins with the premise that a Natural Law must exist, as humans did not invent it, but humans respond to it and cannot escape its influences. From this he proposes that God must exist, and that this God must be made up of three parts: the Father, the Son and the Holy Ghost (or Spirit). Love springs from the relationship between the Father and Son. The Holy Ghost is a conduit for the divine love, and the Son has the responsibility to bring as many human souls to the Father as possible.

Lewis describes what a Christian believes and how a Christian should behave. The beliefs follow the logical argument, and the behavior involves several virtues that should be practiced, with the help of Jesus Christ. The first steps in being a Christian are to accept Jesus Christ and to try to be more like Him. The more closely one can be more like Jesus Christ, the better. However, one does not, and probably cannot, achieve perfection before death. This is acceptable, according to Lewis, and God looks with more favor upon those who struggle harder to be Christian.

There is no other way to everlasting life than through the help of Jesus Christ, and there is no other correct religion. However, Lewis allows that people of other religions or who have never heard of Christianity could be in the process he describes anyway. He extends this possibility to any intelligent being in the universe.

Ultimately, Lewis sees becoming Christian and achieving everlasting life as next step in human evolution. He does not expect any further biological evolution. As a result, new men walk the earth, those who are true Christians and who have achieved a level of perfection. Lewis recognizes these new men and suspects that they recognize each other.

The book consists of four sub-books:

Book I: Right and Wrong as a Clue to the Meaning of the Universe

Book II: What Christians Believe

Book III: Christian Behaviour

Book IV: Beyond Personality: or First Steps in the Doctrine of the Trinity

Final perfection of the human soul occurs after death in a kind of perfection training camp lead by Jesus Christ. Once perfect, Jesus takes a soul to the Father. What happens then is out of Lewis's scope, as it is for every human being. Only the first part of the plan, the part that directly involves human beings, is revealed to humanity.