A person can be an expert in a field but not be able to teach it. Often times they can't pass on their knowledge in a manner that the students can absorb. Certified teachers have expertise in how to teach what they know. (Needless to say, not all of them really have the expertise that they were trained to have. )

“In a time of deceit telling the truth is a revolutionary act.”
― George Orwell

It's not over when you lose. It's over when you quit.