Godly Servants

The Great Commission included instruction for the Apostles to teach their disciples to obey everything Jesus commanded them.  If everything Jesus commanded them had been included in the Gospels, there would be no need for the letters that make up the rest of the New Testament.  But so much of the New Testament are letters from Paul!  Who's Paul?  Should we listen to him?  If he is a legitimate Apostle, that means we have to obey everything he taught!  From Titus 1:1-4.