This is a seldom asked question. Most often the question is WHO! Who am I? I am a mechanic, a lawyer, doctor, janitor, housewife, judge, teacher, mother, etc. And, all of those WHO that I am can change. Many are temporary. I was an electrician, but NOW I am retired. Bill Clinton WAS the president of the United States; however, today he is someone else. His WHO has changed. But, the WHAT is a totally different bird.
WHAT I AM is the eternal question of all time. Of course one cannot miss the I AM-ness of the question. I AM resonates from Exodus 3:14 and John 8:58 with a vibration that cannot be missed. It is not quite as simple as Rene Descartes depicted it to be when he said, “I think, therefore I am.” Plato was a little closer with his dictum, “Know thyself”.
What I am runs a long side of, and conjures up such questions as: is light a wave or a particle, is the universe expanding or falling back in on itself, and what’s the nature of God? It’s seems crazy to even ask such questions, but isn’t that the nature of man: to ask and seek? Knowing God intimately is alow different than reading about God! Knowing the heart of God, while seemingly impossible, nevertheless calls me.
Beyond theism is a term that describes the journey I now find myself travelling. I no longer see God as hiding out somehwere beyond one of those planets biding His time until he signals to Jesus to “head em up, and move em out”. It’s second coming time!
What if the sending of the Spirit was that- the second coming?
I know, now I am rattling your religious paradigms. But, since mine had been rattled, why not yours as well? The good news for you is that you have a choice. You don’t have to read these commentaries. Me? I can’t stop my pusuit of KNOWING GOD and therein finding out WHAT I AM. And for some reason, I believe this hallway is not a short one. 
What have you read that could back up the possibility of Pentecost being the second coming?