First, he is an intolerant bigot. Second, he has no apparent knowledge of the Constitution of the United States. Prove me wrong.
In the meantime, while you're working on that challenge, check out this New Republic article on the vast difference between what Ben Carson believes and what the Constitution actually says. Here are snippets.
Dr. Ben Carson excels in addled interpretations of America’s founding principles. In May, the Republican presidential candidate claimed that the president has the power to ignore the Supreme Court’s gay marriage ruling. And last month, when asked by Meet the Press’ Chuck Todd whether the Bible has "authority" over the Constitution, said, "That is not a simple question." He extended this streak of misinterpretation on Sunday when Todd asked him whether he thought "Islam is consistent with the Constitution." Carson replied, "No, I don't, I do not," and then added, "I would not advocate that we put a Muslim in charge of this nation. I absolutely would not agree with that."
The larger issue, however, is the tendency of many Americans these days, both in and out of politics but especially conservatives, to evoke the Constitution without having any idea what it says or does not say. Even worse, they use a document whose sole purpose was to guarantee freedoms to attempt to try to limit the freedoms of those with whom they disagree. The Constitution is imperfect, of course, and in practice has been used to validate some terrible injustices—slavery, the deportation of Japanese-Americans, or speech that some found politically offensive. But past sins in no way means that we should condescend to our worst instincts. The Constitution can also be a tool to create a society where any American can grow up to be president, even a former neurosurgeon who seems to have little respect for its spirit.
A test for fitness as a candidate for high office should not be membership in any religion. Nor should skin color be a criterion. Regardless of Carson being black and christian, he is not suited for the presidency (or any other political office). And that would still be true if he were white and muslim.
If candidates like Carson can’t be bothered to read and understand the 4,500 words that comprise our founding document, they should not be considered fit for the job that requires they defend it.