What is Pentecost? The facile answer that it marks the gift of the Holy Spirit to the Apostles isn't sufficient.
Why? The Holy Spirit is given and was given many times.
What makes this gift so significant?
It is the answer to the question as to why it is better that Christ depart, as He indicated in John 16:17:
But I tell you the truth: It is for your good that I am going away. Unless I go away, the Counselor will not come to you; but if I go, I will send him to you.
Nothing seems like a good trade when you are trading away Christ, of course. But we should get a hint from another of Christ's enigmatic statements to the Magdalene in the same Gospel at 20:17:
"Do not hold on to me, for I have not yet returned to the Father."
Have not some of you thought the fair but blasphemous thought: I'd rather have Jesus than the Spirit? I know I have!
But let us take Him at His word. The gift the Spirit at Pentecost is the gift of the full power of the Paschal Mystery, which was never given before. Augustine understood the difference made through the Paschal Mystery with the simple nomenclature, 'the grace of Christ.' It stands to reason, then, that the difference between the Pentecost grace and all preceding graces would be the difference between sanctifying grace (that which makes one worthy of heaven) and actual graces (graces that do something, but do not give salvation itself). I think that this was the gift of sanctifying grace for the first time.
Yet, let's study for a moment the characteristics of this grace as portrayed in Acts. It is the grace that inaugurates the new age. It is clearly an outward-leading grace: the communicating of the Gospel to all peoples is its most obvious manifestation here. But you'll also note the virtues that suddenly came into being in the hearts of the Apostles that made this communication possible: courage, zeal, wisdom, piety, love, hope, faith.
Evangelizing cyberspace is a part of the work begun at Pentecost. May we do it with all boldness, wisdom, charity and creativity!