Centuries ago in europe, slavery was seen as a job. Because that person would have benefits like living in the owners house and being clothed and fed. So if a person had nothing, this would have been an opportunity to have something. Slavery took a horrific turn once europeans started "taking" africans from their homeland and making them work for almost nothing. So, that's why slavery is seen in a very negative light.
Religious figures would of course talk well of slavery because they were probably slave owners. I mean, if you had dough, you probably owned a slave.