knowing yourself and what you can/cannot put up with (meaning who you are/are not mentally able to work with). Some people are unable to work with Substance Abuse patients, or maybe patients that have abused kids. It also means, knowing when you, yourself need to seek help. Being a counselor is a hard job and sometimes we try to be all things to all people but we're human and each of us have our limitations. Knowing yourself allows you to take care of yourself so that you can provide the best possible counseling to others.
With regard to Chrisitan Counseling - this means that the counselor needs to be aware of his own barriers, feelings and deficits, as not to bestow these onto those he counsels.
Know thyself.
it means 'know thyself' in classical greek.
Socrates encouraged people to know thyself
Know thyself
(Temet nosce - thine own-self you must know)translated in the Matrix as know thyself
Sometimes they became angry because Socrates often showed them that they didn't know what they were talking about.
know thyself
Be always ashamed to find thyself idle
plato to socrates
If by counselor, you mean attorney, you need a law degree. If by counselor you mean someone who gives advice, you need no degree. You do not even need to know what you are talking about as many of these answers demonstrate.
Plato or Socrates