How can you argue for universal human dignity without religious premises--if you are facing a skeptic on the issue?

Are there any "tribal" or "status"  limits to who deserves to be treated as a real human being. Cultures develop strong rules for how you treat someone, but there are all sorts of limits on who gets treated as a real person. Human inequality was thought to be a basic fact of existence. Good citizens of Greece and Rome treated boys of low birth as sexual objects, and dumped them when they were no longer desirable. Christianity played a role in changing this, in that Christians saw every person as someone for whom Christ died, so would it really make sense to believe that Christ died for human garbage? But Christians have not absorbed this message consistently, as some of them owned slaves and treated them like human garbage. We've become convinced, in theory, that every human being is worth of respect, in society as a whole, but if someone needed to be persuaded that all humans deserve respect and there is no human garbage, I am not sure how you'd argue it without religious premises.