Charles Ellicott Commentary Acts 1:8

"But ye shall receive power, when the Holy Spirit is come upon you: and ye shall be my witnesses both in Jerusalem, and in all Judaea and Samaria, and unto the uttermost part of the earth." — Acts 1:8 (ASV)

But you shall receive power.—The use of the same English noun for two different Greek words is misleading, but if “authority” is used in Acts 1:7, then “power” is an adequate rendering here. The consciousness of a new faculty of thought and speech would be to them a proof that the promise of the Kingdom had not failed.

You shall be witnesses to me.—These words, which are apparently identical with those of Luke 24:48, strike the keynote of the whole book. Those that follow correspond to the great divisions of the Acts—Jerusalem (Acts 1:7); Judea (Acts 9:32, Acts 12:19); Samaria (Acts 8); and the rest of the book as opening the wider record of the witness borne to the uttermost parts of the earth.

And this witness was twofold:

1. of the works, the teachings, and, above all, of the Resurrection of Jesus;
2. of the purpose of the Father as revealed in the Son.

The witness was to be, in language which, though technical, is yet the truest expression of the fact, at once historical and dogmatic.