Herein, Peter is referring to a prophecy in Joel, wherein the Lord brings to pass on the day of Pentecost an experience whereby He pours His Spirit on all flesh -- so that young men see visions, and old men see dreams, and on His servants and handmaidens, will He pour out His Spirit so they prophesy. (Joel 2:28) Moreover, at the same time, there will be wonders in heaven and signs in the earth before that great and notable day of the Lord comes.
But Peter (standing in one accord with the eleven) does not stop there. Peter continues, saying, "Ye men of Israel, hear these words; Jesus of Nazareth, a man approved of God among you by miracles and wonders and signs, which God did by Him in the midst of you, as ye yourselves also know: Him, being delivered by the determinate counsel and foreknowledge of God, ye have taken, and by wicked hands have crucified and slain: whom God hath raised up, having loosed the pains of death: because it was not possible that He should be holden of it. For David speaks concerning Him, I foresaw the Lord always before My face, for He is on My right hand, that I should not be moved: therefore did my heart rejoice, and my tongue was glad; moreover also my flesh shall rest in hope: because Thou wilt not leave My soul in hell, neither wilt thou suffer Thine Holy One to see corruption. Thou hast made known to me the ways of life; thou shalt make me full of joy with Thy countenance." (Acts 2:21-28)
But Peter (standing in one accord with the eleven) does not stop there. Peter continues, saying, "Ye men of Israel, hear these words; Jesus of Nazareth, a man approved of God among you by miracles and wonders and signs, which God did by Him in the midst of you, as ye yourselves also know: Him, being delivered by the determinate counsel and foreknowledge of God, ye have taken, and by wicked hands have crucified and slain: whom God hath raised up, having loosed the pains of death: because it was not possible that He should be holden of it. For David speaks concerning Him, I foresaw the Lord always before My face, for He is on My right hand, that I should not be moved: therefore did my heart rejoice, and my tongue was glad; moreover also my flesh shall rest in hope: because Thou wilt not leave My soul in hell, neither wilt thou suffer Thine Holy One to see corruption. Thou hast made known to me the ways of life; thou shalt make me full of joy with Thy countenance." (Acts 2:21-28)
Again, a prophecy from David the psalmist indicating that every prophecy given of the Lord unto the prophets of old will (and does) come to fruition In this case, we see the culmination of old testament prophecy aligning on he day of Pentecost -- which marks the beginning of the new testament church. (Acts 2:22)
It's also critical to note that the Acts 2 message was not just given to one nationality (as some claim), but to all those who dwell in Jerusalem -- including the Parthians, and Medes, and Elamites, and the dwellers in Mesopotamia, and in Judæa, and Cappadocia, in Pontus, and Asia, Phrygia, and Pamphylia, in Egypt, and in the parts of Libya about Cyrene, and strangers of Rome, Jews and proselytes, Cretes and Arabians.
As we read in the scriptures, Peter begins to speak freely, regarding David's prophecy specifically, and of the patriarch David, that is both dead and buried.
In other words, in prophesying, God had swore an oath to David, that from his loins (according to the flesh) God would raise up Christ to sit on His throne. And David was seeing all of this prior to the prophecy being fulfilled. We also see this clarified in the lineage of Jesus, as outlined within the gospels. This same Christ that followed the Israelites through the Red Sea (the Rock which was Christ) was going to become our Saviour, and upon His throne, all the house of Israel would know assuredly that this same Jesus, whom they crucified is both Lord and Christ. (I Corinthians 10:1-4)
That this same Jesus, as both Lord and Christ would reign as King of kings and Lord of lords throughout this New Testament dispensation of grace. The risen Jesus -- Alpha and Omega, the first and the last, the beginning and the ending.
"Now when they heard this, they were pricked in their heart, and said unto Peter and to the rest of the apostles, "Men and brethren, what shall we do?" Then Peter said unto them, "Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost. For the promise is unto you, and to your children, and to all that are afar off, even as many as the Lord our God shall call." And with many other words did he testify and exhort, saying, "Save yourselves from this untoward generation."
Then they that gladly received His Word (Jesus - the Word made flesh and dwelt among us) were baptized: and the same day there were added unto them about three thousand souls." (Acts 2:36-41)
Then they that gladly received His Word (Jesus - the Word made flesh and dwelt among us) were baptized: and the same day there were added unto them about three thousand souls." (Acts 2:36-41)
The key here is the question -- what must we do?
And as we see from Peter's response, we need to believe and understand that this same Jesus (whom all of us at one point or another has crucified in our hearts) needs to repent, and be water baptized in His precious holy name so that He can clothe us in His righteousness, filling us too with His Spirit of love and mercy and grace.
What was the response? The first 3,000 souls saved adhered to the message of the apostles. If we read further in Acts 2, we see the fruits, wherein the people went breaking bread from house to house, ensuring that no one was without. This is the good news gospel of the Lord Jesus -- that He would love us so much that He would become our Saviour. (I Timothy 3:16)
Don't miss out in His wondrous plan of salvation, wherein we will sit with Him in heavenly places forevermore. Stand in the offering plate.