'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, Negros, black as Cain,
May be refin'd, and join th' angelic train.
Color and race are not the criteria for making heaven. Christianity is not meant for people of a certain color or race. It is open to all. All true Christians, regardless of their color or race, should be entitled to live freely (not under slavery), and make heaven. 
 Wheatley makes reference to the then-popular belief among racist Christians that black people are the descendants of  Cain , the world’s first murderer, whom God cursed . Of course, this is an ignorant reading and is even inconsistent with the Bible itself –  Cain’s descendants all died off during the deluge of Noah.
On Being Brought from Africa to America