The New Testament tells slaves to obey their masters (Ephesians 6:5, Colossians 3:22, 1 Peter 2:18) instead of condemning slavery outright — a truly good and wise God would have unambiguously commanded its abolition.

Why people find it persuasive

Given the chance to say 'slavery is wrong, free your slaves,' the New Testament instead gives instructions for slaves to submit and masters to be kind — that reads like accommodation to, not condemnation of, a monstrous institution, and it was used for centuries to justify chattel slavery.

Important distinctions

• • First-century Greco-Roman slavery (varied, often debt-based or war-captive, sometimes with paths to manumission and civic roles) vs. race-based chattel slavery of the transatlantic trade.
• • Practical pastoral instruction to a powerless minority within an occupied empire vs. political revolutionary program.
• • Explicit endorsement of an institution vs. regulation aimed at protecting the vulnerable within an institution the audience had no power to abolish.
• • The household codes' surface content vs. their embedded subversive theological logic.

Christian response

• • The New Testament writers were a small, powerless, often persecuted minority within the Roman Empire with no political mechanism to abolish an empire-wide institution; a call for slave revolt would have triggered brutal reprisal (Rome crushed slave uprisings, e.g., Spartacus, with mass crucifixions) without freeing anyone, while the actual apostolic strategy was to plant a radical theological seed — that master and slave are equally children of God, equally baptized, equally 'one in Christ' (Galatians 3:28) — that historically proved to be exactly the seed abolitionists later drew on.
• • Paul's letter to Philemon is the clearest data point: he sends the runaway slave Onesimus back not with instructions to remain enslaved but urging Philemon to receive him 'no longer as a slave but... a beloved brother' (Philemon 16), explicitly undermining the master-slave hierarchy from within a single, concrete case — about as strong a move against slavery as a letter to an individual slaveholder within the Roman legal system could make.
• • The household codes also strikingly innovate by addressing slaves directly as moral agents worthy of ethical instruction (unusual in ancient household codes, which typically only addressed the male head of house) and by placing unprecedented reciprocal obligations on masters ('do the same to them, and stop your threatening, knowing that he who is both their Master and yours is in heaven, and that there is no partiality with him' — Ephesians 6:9), a genuinely radical claim of moral equality before God between slave and master in a culture where slaves were legally property.
• • Historically, it was precisely Christian theological resources — the image of God in every person, the Galatians 3:28 equality principle, and texts like Philemon — that abolitionists (Wilberforce, the Quakers, many African-American Christian leaders) explicitly invoked to argue slavery was incompatible with the gospel, even while conceding that other Christians wrongly twisted or ignored these same texts to defend slavery; the fact that a text can be misused does not settle what it actually teaches when read in its own context and canonical whole.
• • Race-based, hereditary, chattel slavery of the kind practiced in the Americas is a categorically different and, on any fair reading, more severe institution than most Greco-Roman slavery, and it is that specific institution the New Testament is often anachronistically read as endorsing; nothing in the text sanctions kidnapping people for slavery, which Paul explicitly condemns as a serious sin alongside murder and perjury in his one direct comment on slave-trading (1 Timothy 1:10).

Christian counterresponse

This is a fair and serious challenge, and honest theologians should concede that the New Testament does not contain an explicit universal abolition command, and that many Christians for centuries used these texts (wrongly, by the lights of the text's own equality principle) to defend slavery. The stronger claim being defended is not that the text is beyond critique on rhetorical directness, but that its theological content — equal dignity before God, familial rather than property relations between believers, direct condemnation of slave-trading — is genuinely in tension with slavery's premises and did in fact become, once its implications were taken seriously, the resource abolitionists used; slow historical uptake reflects human moral inertia and self-interest more than the text's inherent meaning.