Religions change over time. Traditional Judaism and early Christianity both allowed slavery and had laws governing it. Today they completely abhor the notion of slavery. The current pope is backing off on Catholicism's stance on homosexuality.
There are different kinds of slavery. Nearly all systems throughout history treated slaves better than American slavery did. Slavery in the law given to Israel back then was not racist, it was not usually permanent (all slaves were freed every 7 years unless they chose to stay), and the slaves still had some rights and protections. So even someone who supported that kind of slavery could still be against American-style slavery.
