...segue from rendering unto Caesar...

This comparison of taxes to debts, as used by Grimké and Swaine, was an important metaphor in the ongoing debate about tax resistance.  If paying taxes to the government is good in and of itself, because God has so decreed it, then taxpaying can't simply be judged according to its consequences.  It is not a donation or a subscription or a contribution that is made voluntarily, but is a duty akin to repaying a debt.  You can't ethically refuse to repay a loan because you think the person you owe money to will spend it unwisely.

Joshua Maule was a Quaker advocate of war tax resistance.  When a general tax was raised by a certain percentage in order to pay for war expenses (with the government explicitly saying this was the reason), Maule refused to pay this additional fraction of his taxes, and insisted that this was the only position consistent with traditional Quaker teaching regarding war.

Maule's critics asked him why he continued to pay other taxes like excise fees, or the remaining percent of the general tax.  Nathan Hall compared Maule's stand to someone trying to avoid drinking intoxicating beverages by, for instance, only drinking 75% of a bottle of 50-proof (25% alcohol) liquor:

We both have a testimony against the use of ardent spirits, but are, being very thirsty, placed in a situation where we can get no water except some that has a small portion of whiskey in it. Being under the necessity of taking something, thee may, by inquiry and calculation, find what proportion of the objectionable article is contained in it, and leave just that much in thy bowl; while my understanding will be that in partaking I partake of both good and bad, and in refusing refuse both. So that with me the question is and has been, not what portion I should pay so much as whether any at all.

Maule responded in this way:

The arguments used in reference to what disposition the officers of the law might make of the money collected appeared to me to be valueless.... I am not accountable for the acts of other men: if I owe a just debt, I must pay it; if the person receiving the money uses it for a bad purpose, the accountability is with him; but if he demand money of me avowedly to be used in any way to the plundering of my neighbor, destroying his property, or taking his life, then if I furnish money thus demanded I become an accomplice in the evil work and accountable for the sin. I consider our civil taxes a just debt that should be promptly paid, but I am satisfied that no human authority has either a moral or a religious right to demand of me money or means of any kind to aid in destroying the lives and property of my fellow-men. 

Samuel Allinson put forward the most forceful attack on the tax-as-debt analogy. First off, he denied that God commands us to give Caesar anything that Caesar is going to put toward a sinful purpose: "if tribute is demanded for a use that is antichristian, it seems right for every Christian to deny it, for Cæsar can have no title to that which opposes the Lord’s command."  Having knocked out the divine pillar upholding taxpaying as a moral duty, he then proceeds to attack the idea that taxation represents some sort of secularly-contracted debt, or part of an implied "social contract." Allinson argues that no Christian would agree to the terms of such a contract:

Every valid contract is voluntarily entered into, and as it is the duty of every one previously to see that his engagement is innocent, so when his promise is purchased by a consideration given it would be dishonest and deceitful not to perform it, the other party having, as it were, deposited so much effects in his hands, which he is to render back according to agreement and when received the receiver has a right to apply it as he pleases without any account to the payer, but in the case of taxes he who gives has a right to call to such an account and therefore seems himself liable for and privy to the application. Every man has or has not given his assent to the government he lives under, in the first, he has formally declared his allegiance thereto, in the latter, that allegiance is implied in consideration of his receiving the protection and benefit of it in the safety of his person and the security of his property, in both, it is no more than to be “true and faithful” which can never mean a compliance with every requisition, for we owe a superior allegiance to the King of Kings, and whenever the requisitions of man run counter thereto we “ought to obey God rather than men” ... We have never entered into any contract, express or implied, for the payment of taxes for war, nor the performance of anything contrary to our religious duties...