Holy Spirit

The union between the Father and Son is such a live concrete thing that this union itself is also a Person. I know that among human beings, when they get together in a family, or a club, or a trade union, people talk about the “spirit” of that family, or club, or trade union. They talk about its “spirit” because the individual members, when they are together, do really develop particular ways of talking and behaving which they would not have if they were apart. It is as if a sort of communal personality came into existence. Of course, it is not a real person: it is only rather like a person. But that is just one of the differences between God and us. What grows out of the joint life of the Father and Son is a real Person, is in fact the Third of the three Persons who are God. This third Person is called [. . .] the Holy Ghost or the “spirit” of God.

- Mere Christianity