'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."
She no longer believes in polytheism (worship of many gods or godesses). She now believes in one supreme God and Savior, Jesus Christ. 
 
 White slave owners taught their slaves that there was a one and only God and his son Jesus Christ, who saved everyone from sin. They encouraged slaves to believe and have faith in God and religiously taught from the Bible which is ironic, seeing as how the slaves were not permitted to read lest they gain knowledge. Only from their masters is where they learned about God and to sing praises to him which to some were the saddest songs. 

On Being Brought from Africa to America