On the day of Pentecost those who believed Peter's message were being saved. That means that Peter did preach a gospel that day and that gospel was not the "gospel of grace." We can know this because before anyone can be baptized with water they must first believe (Acts 8:36-37) and here we read that those who gladly received Peter's words were baptized:
"Then they that gladly received his word were baptized: and the same day there were added unto them about three thousand souls" (Acts 2:41).
We have an uninterrupted sermon preached by Peter on the day of Pentecost beginning at Acts 2:14 and ending at Acts 2:36. In that sermon there is not a word about God's "grace" and there is not a word about the purpose of the Lord Jesus' death on the Cross. Peter used facts of the death and resurrection of the Lord Jesus to prove the deity of Jesus and the fact that he is the promised Messiah. Peter ended his sermon with the following words:
"Therefore let all the house of Israel know assuredly, that God hath made the same Jesus, whom ye have crucified, both Lord and Christ" (Acts 2:36).
Dr. Stanley D. Toussaint, Senior Professor Emeritus of Bible Exposition at Dallas Theological Seminary (Acts 2), writes the following commentary on Acts 2:36:
"Here is the conclusion to Peter's sermon. The noun 'Lord', referring to 'Christ', probably is a reference to Yahweh. The same word 'kyrios' is used of 'God' in verses 21, 34, and 39 (cf. Phil. 2:9). This is a strong affirmation of Christ's deity" (The Bible Knowledge Commentary; New Testament, ed. Walvoord & Zuck, [ChariotVictor Publishing, 1983], 359).
The Jews who believed that Jesus is Christ, God come in the flesh, were "born of God". Dr. Zane Hodges, past Chairman of of the New Testament Department at Dallas Theological Seminary, writes the following in regard to Peter's words:
"Peter concludes his address with the assertion that 'God has made this Jesus, whom you have crucified, both Lord and Christ' (2:36). His hearers then reply, 'Men and brethren, what shall we do?' (2:37). But such a reaction presumes their acceptance of Peter's claim that they have crucified the one who is Lord and Christ. If this is what they now believe, then they were already regenerated on Johannine terms, since John wrote: 'Whoever believes that Jesus is the Christ is born of God' (1 John 5:1; cf. John 20:31)" [emphasis added] (Hodges, The Gospel Under Siege, 101).
Here are the verses to which Hodges makes reference:
"Whosoever believeth that Jesus is the Christ is born of God...Who is he that overcometh the world, but he that believeth that Jesus is the Son of God?" (1 Jn.5:1,5).
On the day of Pentecost those who believed the "good news" that Jesus is the Christ, the Son of God, were "born of God" and saved. This "good news" is the same good news which was first preached to the Jews:
"For I am not ashamed of the gospel of Christ: for it is the power of God unto salvation to every one that believeth; to the Jew first, and also to the Greek" (Ro.1:16).
The "good news" of Christ which was preached to the Jews first was different "good news" which was later preached to the Gentiles.