Acts 2:
14 But Peter, standing up with the eleven, raised his voice and said to them, “Men of Judea and all who dwell in Jerusalem, let this be known to you, and heed my words. 15 For these are not drunk, as you suppose, since it is only the third hour of the day. 16 But this is what was spoken by the prophet Joel:
17 ‘And it shall come to pass in the last days, says God, That I will pour out of My Spirit on all flesh; Your sons and your daughters shall prophesy, Your young men shall see visions, Your old men shall dream dreams. 18 And on My menservants and on My maidservants I will pour out My Spirit in those days; And they shall prophesy. 19 I will show wonders in heaven above And signs in the earth beneath: Blood and fire and vapor of smoke. 20 The sun shall be turned into darkness, And the moon into blood, Before the coming of the great and awesome day of the Lord. 21 And it shall come to pass That whoever calls on the name of the Lord Shall be saved.’ 22 “Men of Israel, hear these words: Jesus of Nazareth, a Man attested by God to you by miracles, wonders, and signs which God did through Him in your midst, as you yourselves also know— 23 Him, being delivered by the determined purpose and foreknowledge of God, you have taken by lawless hands, have crucified, and put to death; 24 whom God raised up, having loosed the pains of death, because it was not possible that He should be held by it. 25 For David says concerning Him:
‘I foresaw the Lord always before my face, For He is at my right hand, that I may not be shaken. 26 Therefore my heart rejoiced, and my tongue was glad; Moreover my flesh also will rest in hope. 27 For You will not leave my soul in Hades, Nor will You allow Your Holy One to see corruption. 28 You have made known to me the ways of life; You will make me full of joy in Your presence.’ 29 “Men and brethren, let me speak freely to you of the patriarch David, that he is both dead and buried, and his tomb is with us to this day. 30 Therefore, being a prophet, and knowing that God had sworn with an oath to him that of the fruit of his body, according to the flesh, He would raise up the Christ to sit on his throne, 31 he, foreseeing this, spoke concerning the resurrection of the Christ, that His soul was not left in Hades, nor did His flesh see corruption. 32 This Jesus God has raised up, of which we are all witnesses. 33 Therefore being exalted to the right hand of God, and having received from the Father the promise of the Holy Spirit, He poured out this which you now see and hear. 34 “For David did not ascend into the heavens, but he says himself:
‘The Lord said to my Lord, “Sit at My right hand, 35 Till I make Your enemies Your footstool.”’ 36 “Therefore let all the house of Israel know assuredly that God has made this Jesus, whom you crucified, both Lord and Christ.”
37 Now when they heard this, they were cut to the heart, and said to Peter and the rest of the apostles, “Men and brethren, what shall we do?”
38 Then Peter said to them, “Repent, and let every one of you be baptized in the name of Jesus Christ for the remission of sins; and you shall receive the gift of the Holy Spirit. 39 For the promise is to you and to your children, and to all who are afar off, as many as the Lord our God will call.”
Then a summary here same chapter:
40 And with many other words he testified and exhorted them, saying, “Be saved from this perverse generation.” 41 Then those who gladly received his word were baptized; and that day about three thousand souls were added to them.
Then we have in Acts 3 the portico of Solomon sermon:
12 So when Peter saw it, he responded to the people: “Men of Israel, why do you marvel at this? Or why look so intently at us, as though by our own power or godliness we had made this man walk? 13 The God of Abraham, Isaac, and Jacob, the God of our fathers, glorified His Servant Jesus, whom you delivered up and denied in the presence of Pilate, when he was determined to let Him go. 14 But you denied the Holy One and the Just, and asked for a murderer to be granted to you, 15 and killed the Prince of life, whom God raised from the dead, of which we are witnesses. 16 And His name, through faith in His name, has made this man strong, whom you see and know. Yes, the faith which comes through Him has given him this perfect soundness in the presence of you all. 17 “Yet now, brethren, I know that you did it in ignorance, as did also your rulers. 18 But those things which God foretold by the mouth of all His prophets, that the Christ would suffer, He has thus fulfilled. 19 Repent therefore and be converted, that your sins may be blotted out, so that times of refreshing may come from the presence of the Lord, 20 and that He may send Jesus Christ, who was preached to you before, 21 whom heaven must receive until the times of restoration of all things, which God has spoken by the mouth of all His holy prophets since the world began. 22 For Moses truly said to the fathers, ‘The Lord your God will raise up for you a Prophet like me from your brethren. Him you shall hear in all things, whatever He says to you. 23 And it shall be that every soul who will not hear that Prophet shall be utterly destroyed from among the people.’ 24 Yes, and all the prophets, from Samuel and those who follow, as many as have spoken, have also foretold these days. 25 You are sons of the prophets, and of the covenant which God made with our fathers, saying to Abraham, ‘And in your seed all the families of the earth shall be blessed.’ 26 To you first, God, having raised up His Servant Jesus, sent Him to bless you, in turning away every one of you from your iniquities.”
Then we have in chapter 4 Peter says this:
8 Then Peter, filled with the Holy Spirit, said to them, “Rulers of the people and elders of Israel: 9 If we this day are judged for a good deed done to a helpless man, by what means he has been made well, 10 let it be known to you all, and to all the people of Israel, that by the name of Jesus Christ of Nazareth, whom you crucified, whom God raised from the dead, by Him this man stands here before you whole. 11 This is the ‘stone which was rejected by you builders, which has become the chief cornerstone.’ 12 Nor is there salvation in any other, for there is no other name under heaven given among men by which we must be saved.”
I don't see how Peter was preaching a different Gospel.
Pentecost was no secret, but rather the DEFINITE fulfillment of PROPHECY. Acts 2:16 with Eph. 3:5; Acts 3:21 with Rom. 16:25; Acts 3:24 with Col. 1:26; Acts 3:25 with Rom. 5:11.
Pentecost was a Jewish Feast associated with ISRAEL'S REDEMPTION and KINGDOM calendar. Lev. 23:15-21; Deut.16:16.
At Pentecost, Israel was not yet set aside and was thus STILL God's channel of blessing to the nations. Acts 2:14,22,36. Acts 3:25,26. Acts 5:32. Gal. 3:28. Eph. 2:13-18.
Israel's PROMISED KINGDOM was being OFFERED during the Pentecostal era. Acts 2:36-40. Acts 3:18-26.
There was ALREADY A CHURCH IN EXISTENCE on the day of Pentecost- a PROPHESIED CHURCH. Acts 2:47(In order for something to be "added to" it HAD to have ALREADY EXISTED). Matt. 16:18,19. Heb. 2:12. Psalms 22:22.
Early Acts is a record of the "Pentecostal Era." Read Acts 2:1-11; 2:12-47; Acts 3:1-11; Acts 3:12-4:22; Acts 4:23-5:11; Acts 5:12-42; Acts 6"1-7 and Acts 6:8-8:1.
Which BTW, in the very next chapter, we have Paul being saved (Chapter 9). Coincidence?