The first line is one of my favorite quotes... what comes after it is pretty good too.
"The only way to have peace is to teach peace. By teaching peace you must learn it yourself, because you cannot teach what you still dissociate. Only thus can you win back the knowledge that you threw away. An idea that you share you must have. It awakens in your mind through the conviction of teaching it. Everything you teach you are learning. Teach only love, and learn that love is yours and you are love." - ACIM