Be ye therefore very courageous to keep and to do all that is written in the book of the law of Moses, that ye turn not aside therefrom to the right hand or to the left; Be ye therefore very courageous to keep and to do, etc. - It requires no small courage to keep a sound creed in the midst of scoffers, and not less to maintain a godly practice among the profane and profligate.
That is written in the book - By the word of God alone his followers are bound. Nothing is to be received as an article of faith which God has not spoken.