Some view our sable race with scornful eye, "Their color is diabolic die." (5-6)
Through the white looking-glass, this speaker is now looking at herself from a "white" perspective. She refers to her race as "sable," which means both shrouded in black cloth for funerals (death) but also refers to the valuable pelt of the sable animal. She could be suggesting both—that her identity is trapped in the perspective of different races. Whites may view slaves with scorn, while she may find her race beautiful and valuable. Either way, this couplet is where she shows us her ideas about herself through the perspective of society.
The quote is another way to give us an "outsider" perspective. The speaker's identity as black is referred to as "diabolic" or evil. This goes along with the darkness of a "Pagan" land, but it also refers literally to her skin color. It's as if the speaker sees herself through the eyes of others. Maybe that's because they've had so much influence and power over where she ended up (America) that they have some say in her identity.
Remember Christians, Negroes, black as Cain, (7)
Finally the speaker addresses all other "Negroes," as well as Christians. She's including herself here, as one among many. Just like she's been converted in the above lines, her sense of identity is now broadened to be included with all Christians and all Negroes. Why does that matter? Because her sense of self has been morphing throughout the poem. First we saw her associated with her land, then her conversion, then through the eyes of a "scornful eye," then as one among many other Negroes and Christians. Her sense of identity is like a mirror with three sides that shows us different points of view, but all of the same person. Trippy.