The Substance of Belief

Suppose that you claim to believe some given statement of the world and someone responds that you "don't really believe that;" is there a place for such a response? Can you dispute whether or not someone really believes something. Not as in the case of someone who is being knowingly insincere about their beliefs, but someone who believes that they are sincere in what they claim to believe.

It seems to me that the substance and weight of a belief lies in the actions of the believer. If a person claims to believe that, say, all humans are equal and then proceeds to consistently bestow more time and value on one gender or one race over another, we might say that his belief lacks substance. And if a man believes that he is sinning and believes that the consequence for sin is hell and believes that hell is a terrible horrifying place, then either he must discontinue sinning (or actively struggle against his sin, since, perhaps it is a matter of weakness rather than will) or we must say that one of his beliefs lacks substance.