20 Besides the four Gospels, the canon of the New Testament also contains the epistles of St. Paul and other apostolic writings, composed under the inspiration of the Holy Spirit, by which, according to the wise plan of God, those matters which concern Christ the Lord are confirmed, His true teaching is more and more fully stated, the saving power of the divine work of Christ is preached, the story is told of the beginnings of the Church and its marvelous growth, and its glorious fulfillment is foretold. For the Lord Jesus was with His apostles as He had promised (see Matt. 28:20) and sent them the advocate Spirit who would lead them into the fullness of truth (see John 16:13).
Source: Dei Verbum (Vatican.va)