Acts 2:16

This is that. Peter makes no timid approach to his subject. He boldly identifies the preaching as a fulfillment of prophecy. He can afford to do this, since he has been taught by the Lord and is inspired by the Spirit. Under this twofold guidance he begins the first public exposition of the life and works of the Messiah since His ascension. The verses that follow amply testify to the apostle’s new-found ability.

Joel. Peter does not plunge into controversy about Jesus. He first uses the OT Scriptures, which they believed, to sanction the phenomenon they are even then witnessing. This gains their attention, helps them accept his reasoning, and prepares them for his later references to Christ. The prophecy of Joel is thought by some to be the oldest prophetic book of the Bible (see Vol. IV, pp. 20, 21). Under the theme of the coming “day of the Lord” Joel calls Israel to repentance, and promises an outpouring of the Spirit at some future time, identified only as “afterward” (see on Joel 2:28-32; see Vol. IV, pp. 937, 938). The expectation of such an outpouring of the Holy Spirit was strong among devout men of the OT (see on Acts 2:3). Verses 17-21 are quoted from Joel 2:28-32, following quite closely the LXX.