How do we know the letters of the New Testament are authoritative?

The letters of the New Testament are considered authoritative because they are written by apostles who were entrusted with the teachings and mission of Christ. These writings are intended for the church and are meant to be read aloud in a communal setting. They speak to the issues faced by the early church and instruct believers on doctrine, ethics, and the organization of church life. The Apostle Paul explicitly mentions his authority in his letters as one who is called by Christ, thus solidifying their theological importance and applicability to all Christians.