Compare John 5:47 and 6:30. If I have compared correctly, I understand the Lord to say that when one believes that one obeys the Lord from the heart (John 3:36; Romans 6:16-17). Those who do not obey do not believe. This shines a light on those who have expressed belief, but have not obeyed the Lord. Those who live this way call the Lord a liar by the life lived (John 3:33).
So empty it is when people say they believe in Jesus, but they refuse to hear what He says about the life that is to be lived.