How was religion used to justify imperialism

What was the main motivation for the early populists

Which of the following answers best describe why the fugitive slave act was so controversial? a fugitive slaves who were caught were to be sent back to africa, and southern states disagreed with this practice. b it required all runaway slaves to be sentenced to death, and most people were against this extreme form of punishment. c fugitive slaves were required to work twice as long before they could be freed, and northern states opposed this. d it required all americans to return runaway slaves to their owners, and abolitionists in free states opposed this law.

This is for my social studies guide and i can answer 21/22 of it perfectly fine. but can someone tell me what the caste system was, and what divisions/classes that it was apart of it?