The moral of being civil is only ceremonial?
"Although the Law given from God by Moses, as touching Ceremonies and Rites, do not bind Christian men, nor the Civil precepts thereof ought of necessity to be received in any commonwealth; yet notwithstanding, no Christian man whatsoever is free from the obedience of the Commandments which are called Moral." (from Art. 7 of The Thirty-Nine Articles)
The threefold division of the Law is useless as an instrument in the historical exegesis tool box but, then again, it was never designed for that purpose. The threefold division was developed as a hermeneutical tool to discern how Christians ought to respond to the Law.
The Gospels testify that Jesus does not denigrate the Hebrew Scriptures: "Do not think that I have come to abolish the Law or the Prophets" (Matth. 5:17). Indeed, "it is easier for heaven and earth to pass away than for one tiny stroke of a letter in the law to become void" (Luke 16:17). But then he says things which imply that all foods are made clean (Mark 7:19), thus rendering the food laws void.
The early Christian disciples had to wrestle with this and the decision of the Council in Jerusalem (Acts 15) marks one step. The other writings in the New Testament establish that those who are baptised into Christ are to live for God in a way which fulfils the Law but without being under the Law.
Beyond this, there is little agreement. For a general discussion, see, e.g., Daniel Block's article “Preaching Old Testament Law to New Testament Christians,” Hiphil 3 (2006).
In my view, it is better to think of three and more uses (functions) of laws than to divide up the Law into various kinds of precepts, some binding, others not. This allows for any regulation to have a moral, a civil, and a ceremonial intent (and probably a few others). The Sabbath law is a case in point. There is no need to be legalistic about this but it is surely worth having some of the following thoughts and questions about any law in the Torah:
(1) What does this precept tell me about God? His will? His character? How He wants to be approached by His people?
(2) Of which attitudes and actions should I therefore repent and in which am I to delight?
(3) What kind of a man is Jesus, given that He put himself -for a time- under the Law which includes this regulation?
(4) What kind of a society would Israel have been, if they had actually kept the Law, which includes this regulation?? And how might this vision inspire our own society-building?
At its best, the threefold division is a way of acknowledging that the Law teaches us about God's unchanging character and His claims on human behaviour (moral), that it teaches us about God's way of ordering society at a particular time in history (civil), and that the old sacrificial system has been phased out (ceremonial). At its most simplistic, this can lead lead to the rule of thumb which equates moral with eternally binding (God's character is unchanging), civil law with being culturally relative (the implementation on God's will for societies may look differently at different times), and ceremonial with irrelevant.
But the ceremonial laws may still be able to teach us about God's character and the moral law is not legally binding on those who have died with Christ. No, it is much better to think of different uses of the Law - not least because in any disagreement (Sabbath, usury, homosexual practice) people will argue back and forth in which category to put a law...
So what about the prohibition on usury? Its first intent was to prevent taking advantage of the misfortunes of another (Exod. 22:25; Lev. 25:23-38) and thus it speaks to us of a God who cares for the downtrodden and poor and encourages us to imitate him in that. This may be called the moral aspect in traditional language but we should not overlook the theological aspect, what the law tells us about God, about Jesus. But the law was also designed to shape a certain polity and thus has a civil, communitarian aspect. The prohibition in Deut. 23 suggests that there was no place for commercial loans on interest within this polity. A society which were to abide by the regulations in Deut. 23 would prioritise household units and collaborative, risk-sharing ventures, while allowing for the possibility of different socio-economic arrangements with outsiders. This aspect of the law does not seem to me irrelevant for us.
Is there a ceremonial aspect as well? Maybe. "When you come together, it is not the Lord's Supper that you eat, for in eating everyone proceeds with their own supper, so one goes hungry, another gets drunk" (1 Cor. 11:20-21). As we come together, we come as God's debtors, as those who rejoice in the fact that God in Christ has demonstrated to us his love, charity and grace. Can I worship such a God standing next to someone whose misfortune, consequent debt and current interest payments benefit me? Maybe not.