Bible
Why did God allow slavery in the Bible?
The Bible describes several different practices modern English lumps together as "slavery." Some were closer to time-limited debt service, with release built in. Others—especially permanent slavery of people from other nations—are described in ways many careful Christian scholars find genuinely troubling.
The law does limit some of these practices. It bans kidnapping people into slavery outright. But regulating something isn't the same as approving of it. That doesn't erase how serious this is. "It wasn't as bad as later slavery" isn't a complete answer either. Some of it was still people treated as property.
Christians generally read the Bible as describing a broken ancient world honestly, not blessing everything in it. Some laws pushed that world toward more limits and mercy than was common at the time. That's a real response. It isn't a full resolution.
Bible quotations in these answers are generally taken from the Christian Standard Bible®, Copyright © 2017 by Holman Bible Publishers. Used by permission. Christian Standard Bible® and CSB® are federally registered trademarks of Holman Bible Publishers. Some biblical wording may be paraphrased or follow other translations.