But you shall receive power (ability, efficiency, and might)
when the Holy Spirit has come upon you, and you shall be
My witnesses in Jerusalem and all Judea and Samaria
and to the ends (the very bounds) of the earth.
In the verse just prior to this one, Jesus has given a mild rebuke to the apostles for asking Him when He was going to re-establish and restore the kingdom to Israel. His answer is summed up in this paraphrase ... It is not for you to be concerned about the times, year or seasons which the Father has fixed according to His own plan. The Lord then goes on to give them this powerful prophecy ... But you shall receive power when the Holy Spirit has come upon you, and you shall be My witnesses in Jerusalem and all Judea and Samaria, and to the ends of the earth. WOW!
We must remember that most of the apostles were simple fishermen and had never ventured much beyond their villages before they met Christ. Imagine what they thought when, first, He appeared to them after His resurrection, and then once they are over the shock that He has risen, He is telling them to preach the Gospel in places that stunned them.
The Lord first mentions Jerusalem. What do you think they thought about re-entering the city they had just escaped from? After all, it was in the city of Jerusalem that Jesus was tried and crucified. Neither the Jews, nor the Romans would be welcoming them back.
Next, Jesus tells them they will share the Gospel in Samaria. "But wait!", they thought. "Didn't you tell us earlier not to preach to the Gentiles?" But here's the understanding of this change in their instructions ... when the Apostles began their mission of sharing what Jesus had taught them, they were not fit in their abilities to spread the Word. They had not yet received the Holy Spirit, along with (as the Amplified version tells us), the ability, the efficiency, and the might (power) that would be necessary to preach effectively. Secondly, it was part of God's plan that the Gentiles must not have the Gospel brought to them, until the Jews had refused it. This had certainly happened by this point in the timeline. The revelation of Jesus as the Son of God was completed, and the Jews rejected Him. It was time to teach the rest of the world about Him.
And that's where "all Judea and Samaria, and to the end of the earth" come in. They would be instruments of God to spread "The Good News" to the ends of the world --- first to the world, as they knew it to exist; and then because of their work and influence, it would spread throughout the coming centuries to every corner of the earth, as God created it.
But Jesus makes it clear that they will need the power of the Holy Spirit to accomplish their task. Just as He used His power to heal the sick, and cast out demons, in order to declare His message was from the Kingdom of God, He will enable the Holy Spirit to give them that same power to confirm that the doctrine they preached was from God. Just as Jesus needed to distinguish Himself from the false prophets and teachers of the day, so shall they be able to separate themselves from the counterfeit teachers. They will have the same power to do the Father's will.
But we must be clear about what being Christ's "Witness" entails. Yes, they would carry His literal message of redeeming love to those who would repent and have faith in Jesus as their Savior. They would testify as to His words and His deeds, and above all as to His resurrection and ascension. They would offer His saving grace to all who would believe in Him. But "Witness" also means “one who bears witness by his death” … a martyr. They would hold fast to the Truth of who Jesus was and the Truth of His message --- even unto death.
The Bible records this most sobering meaning of what it means to be a witness. It is clearly spelled out in the account of Stephen's death by stoning in Acts 22, and in the recounting of Antipas, whom Jesus calls, "My witness, my faithful one, who was killed (martyred) in your midst" (Revelation 2:13).
So, I wish for us all to realize that when we accept Christ as our Savior and we receive the Holy Spirit, that we receive the sufficiency to preach the Gospel to a lost world, along with the powers to testify to His Kingship. That's a serious responsibility. And we are called to be witnesses ... in every sense of that word. Do you take your task seriously? Are you prepared (and willing) to carry it out to its ultimate end? Jesus did, and He did it for us. As co-heirs in the Kingdom of God, we should do no less.