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).
From this we can know for sure that another gospel was preached on that day and it is not difficult to understand what that "good news" was. 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. Even today those who believe that truth receive "life" through His name:
"Jesus performed many other signs in the presence of his disciples, which are not recorded in this book. But these are written that you may believe that Jesus is the Messiah, the Son of God, and that by believing you may have life in his name" (Jn.20:30-31; NIV).
Charles Ryrie, professor emeritus at Dallas Theological Seminary, says the following about the theme of Peter's sermon on the day of Pentecost"
"To us today it does not mean much to say that Jesus is Christ or Messiah. To a Jew of that day it was an assertion which required convincing proof, and it was the theme of Peter's sermon. Peter's proof is built along very simple lines. First he paints a picture of the Messiah from the Old Testament Scriptures. Then from contemporary facts he presents a picture of Jesus of Nazareth. Finally, he superimposes these two pictures on each other to prove conclusively that Jesus is Messiah" [emphasis added] (Ryrie, "The Significance of Passover," Bibliotheca Sacra, Oct. 1955, Vol.112, # 448, 335).
There can be no doubt that on the day of Pentecost Peter did not preach the "gospel of grace" but instead he preached the truth that Jesus is the Christ, the Son of God. And all those who believed that gospel were saved.
Not long after Paul was converted on the Damascus road he preached the following message to the Jews:
"And straightway he preached Christ in the synagogues, that he is the Son of God.....proving that this is the very Christ" (Acts 9:20,22).
At that time Paul had not yet received the gospel which he was to preach among the Gentiles. He wrote the following:
"But when God, who set me apart from birth and called me by his grace, was pleased to reveal his Son in me so that I might preach him among the Gentiles, I did not consult any man, nor did I go up to Jerusalem to see those who were apostles before I was, but I went immediately into Arabia and later returned to Damascus" (Gal.1:15-17; NIV).
When Paul received a gospel from the Lord Jesus on the Damascus road he immediately went to Damascus (Acts 9:6-8). But when he received the gospel which he preached among the Gentiles he went immediately into Arabia. That can only mean that two different gospels were preached during the Acts period.