7Only be strong and very courageous, being careful to do according to all the law that Moses my servant commanded you.Do not turn from it to the right hand or to the left, that you may have good success wherever you go.
Joshua must show complete obedience (Deuteronomy 26:14). Not turning to the right hand or to the left is a merism, where contrasting parts express totality.
7 Only be strong and very courageous, being careful to do according to all the law that Moses my servant commanded you. Do not turn from it to the right hand or to the left, that you may have good success wherever you go.