This is probably not what you are thinking, but C.S. Lewis said, "Any fool can write learned language: the vernacular is the real test. If you can't turn your faith into it, then either you don't understand it or you don't believe it."
Similar and it's the gist of it. Understanding something well enough to teach it to someone who doesn't, some people seem to be able to learn enough to get by but not teach others what they are supposed to know.