Jesus asked the disciples after His resurrection to wait in Jerusalem for the promise of the father. On the day of Pentecost, the promise of the holy spirit was given to them, immediately they received power to witness for Jesus. Imagine Peter who denied Christ thrice, even before the slave girl, was now bold enough to speak of Him to great multitudes.
You cannot effectively do the work of God in the power of the flesh, you too need the power of the holy spirit. The holy spirit is the custodian of the exploits of the early church. He guides us to the field ready for harvest, He brings the voice of the father to our spirit and He carried out series of functions in the life of the believer and this makes victorious living a reality for us.
In my former book, Theophilus, I wrote about all that Jesus began to do and to teach
2 until the day he was taken up to heaven, after giving instructions through the Holy Spirit to the apostles he had chosen.
3 After his suffering, he presented himself to them and gave many convincing proofs that he was alive. He appeared to them over a period of forty days and spoke about the kingdom of God.
4 On one occasion, while he was eating with them, he gave them this command: “Do not leave Jerusalem, but wait for the gift my Father promised, which you have heard me speak about.
5 For John baptized with water, but in a few days you will be baptized with the Holy Spirit.”
6 Then they gathered around him and asked him, “Lord, are you at this time going to restore the kingdom to Israel?”
7 He said to them: “It is not for you to know the times or dates the Father has set by his own authority.
8 But you will receive power when the Holy Spirit comes on you; and you will be my witnesses in Jerusalem, and in all Judea and Samaria, and to the ends of the earth.”