The quotation you are asking about is not from a poem by Du Bois but from the book A Return to Love by Marianne Williamson:
Our deepest fear is not that we are inadequate. Our deepest 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, gorgeous, talented, fabulous? Actually, who are you not to be? You are a child of God.
Williamson's book was published in 1992 by HarperCollins.
These inspirational words were also used in the movie "Coach Carter," and Nelson Mandela quoted them in his 1994 inaugural speech. Since then, many people have mistakenly attributed the words to Mandela. I can find no source that cites Du Bois as the author.
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