Have you ever thought about the mighty difference in the Twelve Apostles before and after they had the Holy Ghost fall upon them? And the change was immediate, not gradual! Before that day we have no evidence at all of any powerful sermons or any other signs of great leadership. The leaders of the Jews must have been absolutely convinced that killing Jesus would be the end of their problem.

And then the day of Pentecost! Imagine the horror of the ruling council of the Jews when they got word that men of power were appearing on the right and on the left. Suddenly it must have appeared to them as if putting Jesus in the earth was like the planting  of a seed that now sprang up with great yield.

But what did the Holy Ghost do to the Twelve Apostles that not even Jesus could do? Jesus did have one obvious limitation (in this respect); he had a physical body of flesh and bones! It is quite clear that Jesus, with his body, could not enter into the bodies of the Twelve Apostles in order to give them the strength they needed, from within.

But why was it important that the strength given came from within the Apostles, such as the Holy Ghost could give, and not from the outside of their spirit and bodies, such as Jesus had given for the past three years? In order for one Apostle to act with the power of God, one member of the Godhead needed to be present, being in collusion with the spirit of the Apostle in question.

Since Jesus, because of his body, could not enter into the Apostle to be in collusion with his spirit, and since Jesus could only be in one place at the time, the Apostles needed a member of the Godhead who did not have a physical body, and whose influence could be made manifest everywhere at once. The Holy Ghost has those characteristics. By the power of the Holy Ghost, the Apostles could act as if they were gods themselves.

Then, off and on, the Holy Ghost could withdraw somewhat, leaving the Apostles to gradually become the powerful men that the Holy Ghost helped them appear to be.

"Nevertheless I tell you the truth; It is expedient for you that I go away: for if I go not away, the Comforter will not come unto you; but if I depart, I will send him unto you" (John 16:7)

It is clear that Jesus saw that the absence of the Holy Ghost from their souls was a very real problem for the development of the Apostles. He knew he had to leave them in order for them to reach their full potential.