Hi there, Thomas Carlyle was a very influential 19th century social commentator and philosopher. He was quite radical, and often wrote things that many people disagreed with. In this quote (From a book called Heroes and Hero-Worship, he states that a man who is unable to appreciate 'greatness' in others is intrinsically without value himself. It's quite a common Victorian attitude, reflecting a belief that 'great men' were better than people with everyday lives. Disbelief is used in its transitive sense: not to believe or credit (something or somebody).

Similar to the idea in the 中庸 it's saying that a man who doesn't know how to appreciate the great men of the times, is himself a man without value. And the proof of it, is precisely his incapacity to appreciate great men and to draw an example from them.