Here are some important passages from Calvin to add to the two kingdoms files. In his commentary on 1 Cor. 14, especially having to do with order, he raises the question of uniformity when it comes to ecclesiastical polity, tradition, and external forms. He writes:
The design of the admonition is this — that they may not, without having any regard to others, please themselves in their own contrivances or customs. And this is a doctrine of general application; for no Church should be taken up with itself exclusively, to the neglect of others; but on the contrary, they ought all, in their turn, to hold out the right hand to each other, in the way of cherishing mutual fellowship, and accommodating themselves to each other, in so far as a regard to harmony requires.
But here it is asked, whether every Church, according as it has had the precedence of another in the order of time, has it also in its power to bind it to observe its institutions. For Paul seems to intimate this in what he says. For example, Jerusalem was the mother of all the Churches, inasmuch as the word of the Lord had come out from it Was she then at liberty to assume to herself a superior right, so as to bind all others to follow her? I answer, that Paul here does not employ an argument of universal application, but one that was specially applicable to the Corinthians, as is frequently the case. He had, therefore, an eye to individuals, rather than to the thing itself. Hence it does not necessarily follow, that Churches that are of later origin must be bound to observe, in every point, the institutions of the earlier ones, inasmuch as even Paul himself did not bind himself by this rule, so as to obtrude upon other Churches the customs that were in use at Jerusalem. Let there be nothing of ambition — let there be nothing of obstinacy — let there be nothing of pride and contempt for other Churches — let there be, on the other hand, a desire to edify — let there be moderation and prudence; and in that case, amidst a diversity of observances, there will be nothing that is worthy of reproof.
Let us, therefore, bear in mind, that the haughtiness of the Corinthians is here reproved, who, concerned for themselves exclusively, showed no respect to the Churches of earlier origin, from which they had received the gospel, and did not endeavor to accommodate themselves to other Churches, to which the gospel had flowed out from them. Would to God that there were no Corinth in our times, in respect of this fault, as well as of others! But we see how savage men, who have never tasted the gospel, (Hebrews 6:5,) trouble the Churches of the saints by a tyrannical enforcement of their own laws. (Commentary on 1 Cor. 14:36)
Initially, this will strike the reader as more or less standard anti-Roman polemic. Older churches may not bind other churches, those who appear in later times and in different regions, to their ecclesiastical law. But what we should notice is that Calvin does allow churches to create their own laws and customs, based on prudence and charity. He does not presume that there is no place at all for human law in ecclesiastical polity. This is something that he says very clearly in his comments on 1 Cor. 11:3. 1
Calvin’s view is that churches can create their own human laws, even in matters of polity, so long as they do not bind the conscience of their members, nor overstep their jurisdictions by imposing those laws on other churches. A few verses later, Calvin provides further theological grounding for this allowance, and it is, as we would expect, tied to his doctrine of the two kingdoms, or two fora:
Here we have a more general conclusion, which does not merely include, in short compass, the entire case, but also the different parts. Nay farther, it is a rule by which we must regulate everything, that has to do with external polity. As he had discoursed, in various instances, as to rites, he wished to sum up everything here in a brief summary — that decorum should be observed — that confusion should be avoided. This statement shows, that he did not wish to bind consciences by the foregoing precepts, as if they were in themselves necessary, but only in so far as they were subservient to propriety and peace. Hence we gather (as I have said) a doctrine that is always in force, as to the purpose to which the polity of the Church ought to be directed. The Lord has left external rites in our choice with this view — that we may not think that his worship consists wholly in these things. (comment. 1 Cor. 14:40)
Notice those familiar terms: external polity, decorum, and conscience. Also notice that “worship” does not consist “wholly” in these things, that is, the “external rites,” but is rather the spiritual essence to which those things are tools and aids. We are free to regulate polity in order to protect propriety and peace, but we cannot and must not use it to bind the conscience of the believer.