{Power} (dunamin). Not the "power" about which they were
concerned (political organization and equipments for empire on
the order of Rome). Their very question was ample proof of their
need of this new "power" (dunamin), to enable them (from
dunamai, to be able), to grapple with the spread of the gospel
in the world. {When the Holy Ghost is come upon you}
(epelthontos tou hagiou pneumatos eph' humas). Genitive
absolute and is simultaneous in time with the preceding verb
"shall receive" (l(886d)psesthe). The Holy Spirit will give them
the "power" as he comes upon them. This is the baptism of the
Holy Spirit referred to in verse  5 . {My witnesses} (mou
martures). Correct text. "Royal words of magnificent and Divine
assurance" (Furneaux). Our word martyrs is this word martures.
In  Lu 24:48  Jesus calls the disciples "witnesses to these
things" (martures tout(936e), objective genitive). In  Ac 1:22  an
apostle has to be a "witness to the Resurrection" of Christ and
in  10:39  to the life and work of Jesus. Hence there could be no
"apostles" in this sense after the first generation. But here the
apostles are called "my witnesses." "His by a direct personal
relationship" (Knowling). The expanding sphere of their witness
when the Holy Spirit comes upon them is "unto the uttermost part
of the earth" (he(9373) eschatou t(8873) g(8873)). Once they had been
commanded to avoid Samaria ( Mt 10:5 ), but now it is included in
the world program as already outlined on the mountain in Galilee
( Mt 28:19  Mr 16:15 ). Jesus is on Olivet as he points to
Jerusalem, Judea, Samaria, the uttermost (last, eschatou) part
of the earth. The program still beckons us on to world conquest
for Christ. "The Acts themselves form the best commentary on
these words, and the words themselves might be given as the best
summary of the Acts" (Page). The events follow this outline
(Jerusalem till the end of chapter 7, with the martyrdom of
Stephen, the scattering of the saints through Judea and Samaria
in chapter 8, the conversion of Saul, chapter 9, the spread of
the gospel to Romans in Caesarea by Peter (chapter 10), to Greeks
in Antioch (chapter 11), finally Paul's world tours and arrest
and arrival in Rome (chapters 11 to 28).