"You mean there's a ghost, and he's holy? Holy shit!" That's how metal lungsman Ronnie James Dio remembered reacting as a child to Catholic teachings about the Third Person of the Trinity.
Most of us remain just as unable to absorb the idea of a triune God -- one of those weird Mysteries like the virgin birth and consuming the actual body of Christ. What is the Holy Spirit, anyway? Clues reside in the words.
Forget about holy ghost, an awkward translation intended to delineate the kind of semi-approachable Guy in the Sky portayed by Renaissance painters. (We've all seen ghosts.) In the Greek of the New Testament, the neuter word for spirit is pneuma, meaning wind or breath; when paired with holy, the phrase is pneuma hagion. At root, a "holy" entity is one set aside for association with God -- something sacred. So holy spirit is sacred breath, the transcendent force by which God brought clay to life.
What a beautiful metaphor. And as usual, dogma ruins it.
Ancient Greek manuscripts have no capital letters. The capitalization of Holy Spirit is an editorial choice reflecting a Trinitarian doctrine, a doctrine unstated in the Gospels and probably adopted as a sop to polytheistic converts.
Another Trinitarian editorial choice is the addition of "the": The great majority of Greek Gospel references to holy spirit omit the definite article, for instance when "a holy spirit" presides over John's baptism of Jesus and when Mary becomes pregnant through "a holy spirit." (Greek has no indefinite article; English-speakers translate one when the definite article is absent.) Contrary to most English translations, the Greek Gospel texts rarely denote "THE holy spirit," and when they do, it's in the sense of "the sacred breath I mentioned earlier." The names God and Jesus, on the other hand, are almost always accompanied by definite articles -- "the God," "the Jesus."
St. Paul's Epistles do often use "the holy spirit" to mean "God's guiding influence in the world after Jesus' ascension." Paul doesn't mean one of three Divine Persons.
A holy spirit or sacred breath is a useful concept, something to keep in mind when our transitory symbols are desecrated. Something eternal.
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Dio, "Jesus, Mary & the Holy Ghost."
Albert Ayler, "Holy Ghost."