Thomas M'Crie's sincerity

The following anecdote is told of Dr. Thomas M'Crie, the biographer of John Knox.

Two students at a Scottish university, one a Christian, the other an unbeliever, got into a conversation. The unbelieving student stoutly maintained that no clergyman, possessed of any mental powers or liberal acquirements, really believed in the truth of what he preached. The Christian student of course objected, mentioning several intelligent ministers who he did not think could be charged with such dissimulation.
"Can you believe that Dr. X. is not a sincere believer in the tenets which he preaches?"
"Oh, he's a man of the world! He cannot believe in them!"
"What do you say of Dr. Y?"
"He is too much of a scientific man to be a believer!"
"Well then," the Christian student asked, "can you say that Dr. M'Crie does not believe in the truths which he preaches?"
The shot hit home. The unbeliever's face fell, and after a pause he replied:
"You have the advantage of me now; I must grant you that Dr. M'Crie would not preach such doctrines if he did not believe them."
Dr. M'Crie's sincerity closed the mouth of the gainsayer.