On the day of Pentecost, Peter stood with the other eleven and preached Gospel of Jesus.
The essence of that preaching is this.
Jesus the Nazarene, a man attested by God.
He was put to death by nailing Him to the cross
But God raised Him from death
God has raised Jesus to life
Jesus is exalted to the right hand of God
The real source of this teaching is seen in Luke 24 ( verses 25-27, 44-48). Jesus explained to His disciples the things concerning Himself in all the Scriptures.
Luke 24:25-27 (NIV)
[25] He said to them, “How foolish you are, and how slow of heart to believe all that the prophets have spoken!
[26] Did not the Christ have to suffer these things and then enter his glory?”
[27] And beginning with Moses and all the Prophets, he explained to them what was said in all the Scriptures concerning himself.
Luke 24:44-48 (NIV)
[44] He said to them, “This is what I told you while I was still with you: Everything must be fulfilled that is written about me in the Law of Moses, the Prophets and the Psalms.”
[45] Then he opened their minds so they could understand the Scriptures.
[46] He told them, “This is what is written: The Christ will suffer and rise from the dead on the third day,
[47] and repentance and forgiveness of sins will be preached in his name to all nations, beginning at Jerusalem.
[48] You are witnesses of these things.
The apostles’ teaching” was really “the Lord’s teaching.” The apostles were teaching others what the Lord Jesus had taught them.

Acts 2:42 church proclaims the very same teaching in every teaching and preaching session.