Mt 16:16, Acts 2:36, Acts 3:18,20, Acts 4:12, Acts 5:29-32
Peter says that the eternal Word of God spoken of in the Old Testament is the gospel he preached. (1 Pt 1:23-25)
1 Pt 1:23 Being born again, not of corruptible seed, but of incorruptible, by the word of God, which liveth and abideth for ever. 24 For all flesh is as grass, and all the glory of man as the flower of grass. The grass withereth, and the flower thereof falleth away: 25 But the word of the Lord endureth for ever. And this is the word which by the gospel is preached unto you.
Peter was the first to understand the gospel that Jesus is the Christ. (Mt 16:16)
Mt 16:16 And Simon Peter answered and said, Thou art the Christ, the Son of the living God.
After Peter believed that Jesus was the Christ, he preached only the gospel that Jesus was the Christ. (Acts 2:36, Acts 3:18,20, Acts 4:12, Acts 5:29-32)
Acts 2:36 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 3:18 But those things, which God before had shewed by the mouth of all his prophets, that Christ should suffer, he hath so fulfilled. 20 And he shall send Jesus Christ, which before was preached unto you:
Acts 4:12 Neither is there salvation in any other: for there is none other name under heaven given among men, whereby we must be saved.
Acts 5:29 Then Peter and the other apostles answered and said, We ought to obey God rather than men. 30 The God of our fathers raised up Jesus, whom ye slew and hanged on a tree. 31 Him hath God exalted with his right hand to be a Prince and a Saviour, for to give repentance to Israel, and forgiveness of sins. 32 And we are his witnesses of these things; and so is also the Holy Ghost, whom God hath given to them that obey him.