Our greatest fear is not that we are inadequate. Our greatest fear is that we are powerful beyond measure. It is our light, not our darkness that most frightens us. We ask ourselves, who am I to be brilliant, fabulous, gorgeous, talented? Actually, who are you not to be? You are a child of God. You're playing small doesn't serve the world. There's nothing enlightened about shrinking so that other people won't feel insecure around you. We were born to make manifest the glory of God that's within us. It's not just in some of us. It's in everyone. And as we let our own light shine, we automatically give other people permission to do the same. As we are liberated from our own fears, our presence automatically liberates others.

-Marrianne Williamson
A Return to Love

Falsely attributed to Nelson Mandela's 1994 Inaugural Address http://www.nelsonmandela.org/index.php/news/article/deepest_fear_quote_not_mr_mandelas/

Also some claim that W.E.B DuBois is the original source, however, not much evidence exist to substantiate this.