ACTS 1:8

"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."

This verse was and is a promise from Jesus when he returned to his disciples following his resurrection. He does not say "you will receive power" but he does say "You will receive power when the Holy Spirit comes on you" The second part of this verse is also a promise as he says "you will be my witnesses". In both places he states "You will". There is much authority and power in those two words. Authority in that the statement is not wishy washy, it is direct and subtle. "You will" point blank. "Power" in being what is behind the statement as he says "You will". He promises, he predicts (or prophesies) he declares, he stands on it. In the last part of the verse it says "in Jerusalem, and in all of Judea and Samaria, and to the ends of the earth" He picks out three places out of the whole world to use as his example. Why? Because it was the regions that they knew.

I broke it down in definition:

But: An alternative given, following a direct statement.
You will: Portrays authority and power. Is a promise. Gives hope.
Receive: Something given by another openly.
Power: A great force controlled by authority.
When the: is a holding statement, used to link two combinations.
Holy Spirit: the Holy Spirit is the sactifier who frees us to lead new lives. (2Cor 13:17)
Comes on you: is a personal statement. Is periodic (comes). To be filled with. And: not completely finished, more to come.
You will: Portray authority and Power. Gives more hope.
Be my witnesses: to be called forth to testify what you have seen and heard.
In Jerusalem: within their midst, among their own.
And in all Judea: A bigger goal, bigger picture, bigger challenge.
And Samaria: Same as above.
And to the ends of the earth: Every unreached people group in the world.