Pentecost may be the most important celebration in the Christian calendar for me. It is also horribly under-regarded. Christianity recognizes Trinity: God interacting with creation in three primary and personal ways. One of those is through creation itself. Christ is another. What about the third?  When Christians talk about "God changing hearts and lives" they are referring to the Holy Spirit.  I've heard some people resist the notion that such a manifestation of deity could be called a person, but this is precisely how a relationship with God actually becomes personal.

Different Drummer: The Holy Spirit