In Michael Shaara's book, The Killer Angels, the lives of several people are followed—their thoughts and actions shared—over the course of four days...
...during which the Battle of Gettysburg, the turning point of the American Civil War, was fought in Pennsylvania.
Both confederate and union leaders are included in this "recreation" of those days. One of the Union leaders is Colonel Joshua Lawrence Chamberlain, who is in command of the Twentieth Maine.
On Thursday, July 2, the second of the four days, Chamberlain's aide Buster Kilrain reports that they have found a black man, wounded, who is unable to communicate with them. The consensus is that this man must be a runaway slave. Regardless of the circumstances that bring him into their midst, he is fed and given medical care. In a conversation between Chamberlain and Kilrain, the colonel notes that he sees no difference between white men and black men. While he admits that he has not known many blacks, he says that when looking into a black man's eyes, he sees a man there (the inference is "not a color")—in those eyes there is "a divine spark." He says that this is something his mother used to say, and he feels that he and the black man share this "divine spark."
My interpretation of this phrase is that for Chamberlain there is a piece of God in all of us. ("Divine" generally refers to God). This spark may perhaps be what gives us life or perhaps it is our soul shining through, but it comes from God and does not distinguish between men based on skin color. I assume the premise is that as God's children, God sees no difference, and neither does Chamberlain. As he and Kilrain continue to speak, Chamberlain goes on to comment that he believes slavery to be "a terrible thing."
"The divine spark" is supposed to be man's conscience. It is that within us that reflects God's desire to "make Man in our own image" (Book of Genesis)