In this chapter Lewis tackles the Christian doctrine of the Trinity: one God in three Persons (Father, Son, and Holy Spirit [or Holy Ghost]). "The most difficult thing about the Christian concept of the Trinity is that there is no way to perfectly and completely understand it. The Trinity is a concept that is impossible for any human being to fully understand, let alone explain. God is infinitely greater than we are; therefore, we should not expect to be able to fully understand Him. . . . The Father is God, the Son is God, and the Holy Spirit is God—but there is only one God. That is the biblical doctrine of the Trinity" (gotquestions.org).
The word "Trinity" does not occur in Scripture. But the idea of a three-in-one God is expressed in passages such as the following.
Matthew 3:16-17 (KJV): "And Jesus [the Son], when he was baptized, went up straightway out of the water: and, lo, the heavens were opened unto him, and he saw the [Holy] Spirit of God descending like a dove, and lighting upon him: And lo a voice from heaven [the Father], saying, This is my beloved Son, in whom I am well pleased."
2 Corinthians 13:14 (KJV):"The grace of the Lord Jesus Christ, and the love of God, and the communion of the Holy Ghost, be with you all. Amen."
Click on the link above for a pretty good discussion of this difficult topic.