I am curious to know how my brother will answer this.

What are the religious "duties" of murderers and thieves, but to repent, to forsake their evil ways at once, and to make lawful reparation?
And what are the "duties" of those whom murderers and thieves assault, but to resist, and to seek the conviction of the evil-doer?
Oh how strange it seems for the Apostle to counsel masters and slaves to imitate their "Master which is in heaven," in their relation to each other, if holding men in bondage be "the sum of all villanies," and how strange for him to send Onesimus back to the system to behave in it as Christ would act in his place! "'Onesimus escapes, we will say, from a gang of murderers, or from a company of thieves, and the Apostle's preaching is the means of his becoming a good man.

Paul writes a letter to the chief murderer of the gang, or to the captain of the robbers, sends Onesimus back, and "beseeches" the brigand for "his son Onesimus," telling him that now he receives him "forever," and then calls the desperado "our dearly beloved fellow-laborer"! Why not, with equal propriety, if slavery be, necessarily, as our brother describes it?
There is some mistake in our brother's theory.
"'I venture to state the distinction which I think he overlooks, and which, if observed, will relieve his difficulty.

Paul never denounces government; "the powers that be are ordained of God." He appeals to "Caesar"; he goes before "Nero"; he never counsels insurrection, nor denounces government, in whatever hands or under whatever forms it may be; but he enjoins principles and duties which, if observed, would make "Caesars," even though they be "Neros," blessings, and their despotisms even would cease to be a curse.

So with slave-holding.