In the interview I saw, Cain was asked about having a Muslim serve in his cabinet. Cabinet positions serve at the presidents pleasure, and he can appoint or not appoint anyone he wants based on whatever criteria he wants (subject to Senate confirmation of course). That is not a religious test for office, anymore than a citizen not voting for a candidate because he is, say Mormon, is, Passing a statute that no Muslim can hold a cabinet position would be.
IF, IF that is the full context of his statements, I agree.
But that's NOT how it appears in the excerpt.