This sermon was presented on October 25, 1780, the day of the commencement of the Constitutional government of Massachusetts, and it can be read online at: https://archive.org/details/sermonpreachedbe00coop. In this sermon, Dr. Cooper compared the republican form of government adopted by Massachusetts and written by John Adams with the form of government which God Himself had given to the nation of Israel. Cooper claimed:
"The form of government originally established in the Hebrew nation by a charter from heaven, was that of a free republic, over which God himself, in peculiar favour to that people, was pleased to preside. It consisted of three parts; a chief magistrate who was called judge or leader, such as Joshua and others, a council of seventy chosen men, and the general assemblies of the people."
Dr. Cooper then proceeded to give several examples to prove his point including the fact that:
"Even the law of Moses, though framed by God himself, was not imposed upon that people against their will; it was laid open before the whole congregation of Israel; they freely adopted it, and it became their law, not only by divine appointment, but by their own voluntary and express consent."
The only defense that your opponent can bring against the claim that Israel had a republican government is the contrary claim that they had a theocratic government. Dr. Cooper adressed this argument directly by saying that:
"the Hebrew government, tho’ a theocracy, was yet as to the outward part of it, a free republic, and that the sovereignty resided in the people."
But this objection can also be answered by turning to John Adams' "Defense of the Constitutions of Government of the United States of America." In this book, Adams pointed out that the ancient republic of Greece as established by the laws of Solon was even more theocratic than Israel, for:
"the title of king was preserved to the high-priest, so the person presiding over the religion of each tribe was called philobasileus, king's friend, and was always appointed from among the nobly born, eupatridæ. Thus religion was always in the hands of the aristocratical part of the community."
Secularists have no problem pointing to Solon's Athens as one of the seeds of American republicanism in spite of that nation's total union of church and state, so they should not have any objection to the claim that Israel, with their removal of the tribe of Levi from any participation in the government, was a republic even though a theocracy in the sense that God was their ultimate authority.
(By the way, you may also want to read my article on the history of the word "theocracy." It is available online at: http://www.increasinglearning.com/american-theocracy.html.)
In addition to Dr. Cooper's sermon, you should also reference Dr. Samule Langdon's sermon entitled: "The Republic of the Israelites: An Example to the American States." This sermon was preached to the legislature of New Hampshire on the day that they ratified the Federal Constitution in 1788. Langdon made essentially the same argument as Cooper did in his sermon, and several additional sermons from that era could be found which echo these. Dr. Langdon's sermon can be found online at: http://thefederalistpapers.integratedmarket.netdna-cdn.com/wp-content/uploads/2013/08/Sunday-Sermon-The-Republic-of-the-Israelites.pdf.
Now your opponent may argue that these two sermons are just examples of attmepts by the clergy to make themselves more relevant to the new secular government, but the problem with that claim is the fact that this view of Israel as having a republican form of government predates the formation of the American governments by nearly 2,000 years. In my essay "We the People: The Biblical Precedent for Popular Sovereignty," I documented the fact that this view is at least as old as Christianity itself. To prove this, I included quotes from Christian authors beginning with the early church fathers, continuing through Augustine and Aquinas, and culminating in the claims of the philosophers of the 17th century.
I have not yet finished part two of that essay in which I will present the fact that every single one of the "enlightenment" authors which influenced the American founders also recognized that Israel had a republican form of government, but you can read about this in a recent book by Eric Nelson entitled "The Hebrew Republic" and available at: http://www.amazon.com/dp/B007IJN3XM. Nelson misunderstood the concept of an agrarian balance and, consequently, falls into the error of claiming that the Bible teaches a system of redistribution, but otherwise, his book is fairly accurate.
To give you just a few examples to pursue for your own research, let me recommend that you read James Harrington's second volume of "The Art of Lawgiving" which he gave the title of: "The Second Book, Containing the Commonwealths of the Hebrews." Harrington was frequently consulted and quoted by the American founders, and he was very explicit in claiming that Israel had a republican government. Harrington wrote that:
“All government is of three kinds: a government of servants, a government of subjects, or a government of citizens. The first is absolute monarchy, as that of Turky; the second aristocratical monarchy, as that of France; the third a commonwealth, as Israel, Rome, Holland.”
“the people of Israel are commanded to take wise men, and understanding, and known among their tribes, to be made rulers over them.”
“For David was a king, who nevertheless did no otherways make any law than by proposition to the people, and their free suffrage upon it. (I Chron. 13.) David consulted with the captains of thousands, and hundreds, and with every leader ... and David said to all the congregation, If it seems good to you ... let us send abroad to our brethren every where that are left in all the land of Israel ... and let us bring the ark of God to us.”
Just to give a few examples.
Additionally, Hugo Grotius once wrote of Israel in his book "The Rights of War and Peace:"
“If, however, there is somehow to be found a republic which could rightly point to the true God as its founder, then this must clearly be the one that all other ones should set themselves to imitate and seek to resemble as closely as they can.”
You should also consider Algernon Sidney's claim that:
"If I should undertake to say, there never was a good government in the world, that did not consist of the three simple species of monarchy, aristocracy, and democracy, I think I may make it good. This at the least is certain, that the government of the Hebrews, instituted by God, had a judge, the great Sanhedrim, and general assemblies of the people."
In fact, Sidney's entire "Discourses Concerning Government" nothing less than a Bible study on the proper nature and role of government. This could also be compared to John Locke's "First Treatise on Government" as well as Montesquieu's "Spirit of Laws," and Sir William Blackstone directly states in his commentaries that:
"Upon these two foundations, the law of nature and the law of revelation, depend all human laws; that is to say, no human law should be suffered to contradict these." (http://www.increasinglearning.com/the-source-of-the-law.html)
At this point, the major argument which I receive from secularists is that all of these philosophers were just giving token adherence to the Bible in order to avoid being persecuted by the church. (Of course, they seldom remember that Sidney was executed for treason after publishing his "Discourses.") They then claim that the founders recognized this subterfuge for what it was and, with the religious freedom available to them in this nation, they were able to form a government based on the purely secular principles that these enlightenment authors were really promoting.
To counter this argument, you could begin with the fact that Benjamin Franklin wrote a letter to Dr. Cooper thanking him for his sermon comparing the republic of Israel with that of the American states and expressing his desire to publish that sermon in Europe for the benefit of freedom loving people there. Here is what Franklin wrote to Dr. Cooper:
"Your excellent Sermon gave me abundance of Pleasure, and is much admired by several of my Friends who understand English. I purpose to get it translated & printed at Geneva at the End of a Translation of your new Constitution. Nothing could be happier than your Choice of a Text, & your Application of it. It was not necessary in New England where every body reads the Bible, and is acquainted with Scripture Phrases, that you should note the Texts from which you took them; but I have observed in England as well as in France, that Verses and Expressions taken from the sacred Writings, and not known to be such, appear very strange and awkward to some Readers; and I shall therefore in my Edition take the Liberty of marking the quoted Texts in the Margin."
The full text of this letter can be found in the Franklin papers online at: http://franklinpapers.org/franklin/framedVolumes.jsp?vol=35&page=068a.
Additionally, when the Federal Constitution was presented to the states for ratification, Franklin wrote a letter the the Federal Gazette in which he argued that those Americans who were opposed to the Constitution were following in the footsteps of those who opposed God's Constitution for Israel. The text of that letter is available on my site at: http://www.increasinglearning.com/blog/franklin-israel.
And Fanklin further evidenced his acceptance of the republican nature of Israel's government when he argued in the Constitutional Convention that:
"We should remember the character which the Scripture requires in rulers, that they should be men hating covetousness."
This particular statement came at the end of a very lengthy debate over the requirements for members of the House of Representatives. There were many delegates to the Convention who argued that the men should be required to own a certain amount of property in America before being permitted to serve in Congress. They claimed that this would give these men a vested interest in passing laws beneficial to American property owners. Others opposed this argument on the basis that it would prevent men who would otherwise make great legislators from being chosen by the people to fulfill that role. This was one of the most heated debates in the convention, and after several failed attempts to resolve it, Franklin presented his argument from Scripture. Madison records in his Journal of the Convention that after Franklin's speech "The motion [to require property ownership] was rejected by so general a no, that the States were not called."
In the closing chapter of my book "Hidden Facts of the Founding Era," I included several additional quotes from founders recognizing that republican government is advocated in the Bible and I provided a point by point comparison of 48 statements in the Constituion which correlate with biblical principles. (http://www.amazon.com/dp/B00ANLTPE8) I've also written an extensive explanation of John Adams claim that America was founded on "the general principles of Christianity." (http://www.increasinglearning.com/general-principles.html) And in case your opponent decides to throw Gregg Frazer's argument at you, you may also want to read my book "The Founders and the Myth of Theistic Rationalism." (http://www.amazon.com/dp/B00BGSM1UC)
I guess I've written enough to keep you busy for quite a while, so I'll go ahead and stop with that. Please do not hesitate to contact me if you have any questions. I have a small network of researchers that I can call on to find any original source documents that you need, and we will all be praying for God to give you wisdom as you prepare for this important event.