Summary: The Holy Spirit transforms the disciples from men of fear to men of faith.
The gospels tell the story of fearful disciples who desert Jesus after his arrest. Peter denies him three times(John 18:25-27), and then joins the other disciples who hide
from Roman soldiers. At Christ's crucifixion, only John stood by the cross of Jesus with the three women: Mary the mother of Jesus, Mary wife of Cleophas, and Mary Magdalene.
But after the death of Jesus, Peter and the others disciples no longer hid from Roman soldiers. The book of Acts tells the story of Peter, standing up with the eleven, lifting up his voice, speaking to the the men of Judaea, and all that lived in Jerusalem (Acts 2:14) The disciples stood up, and Peter preaches up on the day of Pentecost ( Acts 2:14-40). Fear no longer lives in these disicples; fear is cast out now that the Holy Ghost lives within.
Jesus, the Christ, words to his disciples before his ascension to heaven spoke of the power of the Holy Ghost to make the disciples witnessess of him. But ye shall receive power, after that the Holy Ghost is come upon you: and ye shall be witnessess unto me both in Jerusalem, and in all Judaea, and in Samaria, and unto the uttermost part of the earth (Acts 1:8).
The Holy Ghost gives the disciples power to stand up and witness that Jesus is the Christ, Son of the true and living God. Peter, after receiving the Holy Ghost no longer denies Jesus before a few, but is a witness of Jesus before all in the city.