The true church, which started on the day of Pentecost, were the Apostle’s and they preached what we call the Apostle’s doctrine to this day.
Jude, in verse three, admonished all to “contend for the faith that was once delivered to the saints.”
Who delivered this faith?
Peter, having the keys to the kingdom of heaven (Matt. 16:19), delivered the saving message to the Jews and later to the Gentiles (Acts 10).
The Jews, under great conviction, cried out to Peter and the rest of the apostles, “What shall we do?”

Peter said. “Repent and be baptized every one of you in the name of Jesus Christ for the remission of sins and ye shall receive the gift of the Holy Ghost.
The promise is unto you and your children, and to all that are afar off, even as many as the Lord our God shall call” (Acts 2:38, 39)
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) This is the Apostolic faith and doctrine in action. Acts 2:42 – “And they continued steadfastly in the apostles’ doctrine.