Okay, check this out. Phillis Wheatley was born in roughly 1750 and died in 1784. She wrote the following poem:
On Being Brought from Africa to America
‘Twas mercy brought me from my Pagan land,
Taught my benighted soul to understand
That there’s a God, that there’s a Saviour too.
Once I redemption neither sought nor knew.
Some view our sable race with scornful eye,
"Their colour is a diabolic die."
Remember, Christians, Negroes, black as Cain,
May be refin’d, and join th’ angelic train.
I’m not going to get into the sincerity of Wheatley’s religious beliefs. It’s hard to read that opening line now days and really believe that she believed that to be true. But let’s assume it was.
The whole poem then is about how great God is and how great being Christian is and how, when Christian Negroes die, they’re going to go to heaven right along with the white folks.
That’s where I think there’s a little snark going on. The early part of the poem is directed upwards towards God and the last part of the poem casts an eye out to the injustice around her.–"You, who hate us so much, gave us the tools to be your equals in heaven."
As they say on The Simpsons, ha ha.
Tiny Cat Pants 
I put together an open-mic poetry reading at work for Black History Month (I’m white), and this was the second work that I used. The lady who read it did a fantastic job, and she read it with just the inflections that you described. Worked perfectly. I can’t see it meaning anything else!