But ye shall receive power, after that the Holy Ghost is come upon you: and ye shall be witnesses unto me both in Jerusalem, and in all Judaea, and in Samaria, and unto the uttermost part of the earth.
Note 5 at Ac 1:8: The Greek word used for "power" here is "DUNAMIS," and it means "miraculous power" (Strong's Concordance). This word was also translated as "ability," "might," and "strength." It is the root word from which we derive our words "dynamic" and "dynamite." This clearly speaks of the presence and anointing of the Holy Spirit as being the power source in the believers' lives.
It was this anointing power of the Holy Spirit that worked miracles in the life of Jesus (Ac 10:38). We receive the same Holy Spirit power, and we can expect to see it accomplish the same miraculous works that Jesus did (Joh 14:12) if we will believe.
Note 6 at Ac 1:8: The Lord has empowered us to be witnesses, not the judge and jury. Witnesses simply relate what they have experienced, thereby providing evidence to the truth of something. We are to witness in word and deed to the truth of Jesus being alive in us, and let God be the one who convicts (see note 76 at Joh 16:8).
Note 7 at Ac 1:8: Jesus specified an orderly progression in the way we should be His witnesses. First, we start in Jerusalem (i.e., where we live). Then, we go to Judea (i.e., those nearby). Finally, we take the Gospel to Samaria (i.e., every religious and racial group throughout the whole world).
There are some very practical reasons for becoming witnesses in this way. Jesus testified that prophets are honored everywhere except in their hometowns among their families and friends (Mr 6:4). Typically, home is the hardest place to witness. Starting with those who know us best will cause us to humble ourselves and give God all the glory.
Also, if rejection comes, this tempers us and our witness so that we will be more effective and more resilient when we go to the uttermost parts of the earth. People who have a desire for the millions overseas to be converted yet are unconcerned over the condition of their own families and friends, have a problem and will not be the kind of witnesses that Jesus needs. Those who are waiting for a worldwide ministry and are not actively pursuing the opportunities at hand will never be used. We need to be faithful in little before God will give us the "many things" (Mt 25:21).