Why would sewall's "the selling of joseph" have been controversial for his time?

“The Selling of joseph” was written in 1700 and it was the first piece of publication which condemned the slavery in its all forms. Samuel Sewall highlighted the issue in an amazingly captivating way by presenting arguments from liberal point of view. He also presented several arguments from Bible also to reject the idea of slavery.

In his times people used to bring forth many typical justifications for African slavery in North America, Samuel rejected them making the piece controversial for his times.  The last copy of “The Selling of Joseph” is safe at the Massachusetts Historical Society.

yes you could be a life guard if that the question.