This is that which hath been spoken by the prophet Joel (touto estin to eirhmenon dia tou prophtou Iwhl). Positive interpretation of the supernatural phenomena in the light of the Messianic prophecy of Joel 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 (dunami) 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 Joel 17-21 closely the LXX text of Joel and then applies the passage to the present emergency ( Joel 22-24 ).