{This is that which hath been spoken by the prophet Joel}
(	outo estin to eir(886d)enon dia tou proph(8874)ou I(9388)l). Positive
interpretation of the supernatural phenomena in the light of the
Messianic prophecy of  Joe 2:28-32 . Peter's mind is now opened
by the Holy Spirit to understand the Messianic prophecy and the
fulfilment right before their eyes. Peter now has spiritual
insight and moral courage. The {power} (dunamis) of the Holy
Spirit has come upon him as he proceeds to give the first
interpretation of the life and work of Jesus Christ since his
Ascension. It is also the first formal apology for Christianity
to a public audience. Peter rises to the height of his powers in
this remarkable sermon. Jesus had foretold that he would be a
Rock and now he is no longer shale, but a solid force for
aggressive Christianity. He follows here in verses  17-21 
closely the LXX text of Joel and then applies the passage to the
present emergency ( 22-24 ).