Some view our race with scornful eye, (5)
She's not saying who views her race with scornful eye, but we can hazard a guess that she's referring to mistreatment of blacks by whites in America. This may have been to avoid coming right out an accusing anyone of racism, but it's clear that the speaker has experienced prejudice. That's not cool, but she says it in such a matter-of-fact way that it sounds like a simple reality of her life, which faith has made bearable. And that part is kind of cool.
"Their colour is a diabolic die." (6)
So, if someone comes up to you and says, "the color if your skin is evil," you'd probably feel like that person is being unfair, maybe even a tad bit racist, and that they are unfairly judging you by the color of your skin, which you can't really do anything about. And it might even feel worse if the color of your skin is associated with a country from which you've been kidnapped. But now that you're in this new country, which you didn't choose to come to, people think you're evil. And what's even stranger, you've changed religions and now believe that all people are equal and can be saved by God's grace. That'd be complicated, right?
Well, Wheatley accomplishes capturing that complexity in just a few short lines, like the one we quoted. Sure it rhymes with the line that comes before it, but that's the easy part. What's interesting is the word play on the word "die," and that fact that she's putting a quote in her poem. What "die" is she talking about—death or dye? Also, who's speaking? And why, in her poem, does she give a whole line to a racist comment? The unfair treatment that the speaker has received has been turned into a catalyst for her new faith in God and belief in equality of all people.
Remember, Christians, Negros, black as Cain, (7)
Wheatley's getting biblical on us and references Cain as an image associated with blacks in America. But first, she's addressed Christians, and then mentioned "Negros." So what's the big deal? Cain was an outcast after killing his brother. He was marked by God and is generally seen as a character of betrayal, murder, and jealousy. He's diabolic, we might say, and someone to be viewed with scorn—just like the speaker has referred to her "sable race."
The tone has shifted here, though. She's addressing other blacks in America and saying, no matter what you've done, or how other people see you, you can be saved by God's grace. So the prejudice here could be coming from the judgment of others, or the fear people have of themselves after they've done something wrong. Either way, prejudice is out, redemption is in, but it's Wheatley's careful use of reference and imagery that captures the complexity of her experience in America that is so noteworthy.