Charles Ellicott Commentary Isaiah 30:21

"and thine ears shall hear a word behind thee, saying, This is the way, walk ye in it; when ye turn to the right hand, and when ye turn to the left." — Isaiah 30:21 (ASV)

Your ears shall hear a word behind you. —The voice of the human teacher, whom the people looked to as they listened, would find an echo in that inner voice telling them which was the true way, when they were tempted to turn to the right hand or the left.