Acts 2:42
42 And they continued stedfastly in the apostles' doctrine and fellowship, and in breaking of bread, and in prayers.
What is the apostles' doctrine? It is what the apostles taught by the authority of Christ through the inspiration of the Holy Spirit, as recorded in the New Testament.
Ephesians 4: 11-12
11 And he gave some, apostles; and some, prophets; and some, evangelists; and some, pastors and teachers;12 For the perfecting of the saints, for the work of the ministry, for the edifying of the body of Christ: