The Bible regulates and permits slavery rather than condemning it outright — Exodus and Leviticus give rules for owning people, and Paul tells slaves to obey their masters. A book that endorses slavery cannot be the word of a good God.

Why people find it persuasive

Slavery is among the clearest moral evils by modern lights, and texts that regulate it rather than abolish it look like moral endorsement, not accommodation.

Important distinctions

• • Ancient Near Eastern debt-bondage/indentured service vs. chattel slavery of the trans-Atlantic type (permanent, race-based, hereditary, involving kidnapping).
• • Regulation that restrains an existing evil vs. regulation that institutes or commends it.
• • Descriptive law given to a specific ancient nation vs. prescriptive universal moral endorsement.
• • The trajectory of the biblical text as a whole vs. isolated verses read in isolation.

Christian response

• • Exodus 21:16 imposes the death penalty for kidnapping a person to enslave or sell them — this single verse would make the entire trans-Atlantic slave trade a capital crime under Mosaic law, since that trade began precisely with kidnapping in West Africa.
• • Most Old Testament slavery legislation concerns debt-bondage as a form of social welfare in a subsistence economy without bankruptcy law or state relief — Leviticus 25 mandates release in the Jubilee year, and Deuteronomy 15:12-15 requires masters to free Hebrew debt-servants after six years and send them away with a generous parting gift, not empty-handed.
• • Deuteronomy 23:15-16 forbids returning an escaped slave to their master — a direct contradiction of the American Fugitive Slave Act logic that pro-slavery apologists in the antebellum South had to work hard, and dishonestly, to explain away.
• • Paul's letter to Philemon returns the runaway slave Onesimus not as property to be punished but appeals for him to be received 'no longer as a slave but... as a beloved brother' (Philemon 16) — a text that quietly subverts the master-slave hierarchy from within rather than endorsing it.
• • Galatians 3:28 ('there is neither slave nor free... you are all one in Christ Jesus') supplied the theological seed that abolitionists from the Quakers to William Wilberforce to Frederick Douglass — himself a devout Christian — explicitly invoked against slaveholding co-religionists; the abolition movement was substantially a Christian movement arguing from scripture against slaveholding Christians who misused the same text.
• • It remains true and troubling that household codes (Ephesians 6:5, Colossians 3:22, 1 Peter 2:18) instruct existing slaves to obey their masters rather than issuing an explicit command to immediately abolish the institution — the text accommodates a social reality it does not univocally command into being, and Christians should not minimize how badly that accommodation was exploited.

Christian counterresponse

That is a serious and largely fair criticism; defenders typically respond that scripture works pedagogically within history rather than dropping a complete modern ethical code onto Bronze and Iron Age societies overnight, planting principles (the imago Dei of Genesis 1:27, Galatians 3:28) that took time, and courageous readers, to unfold into full abolitionism — but they should concede this is a genuine cost, not a clean answer, and that many Christians used the same texts for evil for a very long time.