I'm working my way through a very interesting book on Calvin's exegesis called The School of God, written by Raymond Blacketer; a man who is apparently a student of Richard Muller. Like Muller, Blacketer pays close attention to Calvin's rhetorical education and in particular to how Calvin allowed that training to aid him in interpreting the Scriptures. In doing so, Calvin was following in a long train of Bible expositors tracing back to the early Church, and most famously Augustine.
One of the key ideas from classical rhetoric that Calvin and most of the other Reformers employed was the notion of commonplaces. Commonplaces are topics upon which, or around which collections of arguments or disputes collect and serve as a stockpile from which the orator can draw to make a persuasive case for some or another proposition. It was Melanchthon who brought the idea of commonplaces into the writing of Christian doctrine in his Loci Communes. Interesting, as we learn from Muller's book, The Unaccommodated Calvin, Melanchthon drew out his commonplaces from Scripture itself. Not Scripture as a whole, but rather Paul's epistle to the Romans, which follows the following topical order:
4. The people of God and the call of the Gentiles
6. Good works
7. Civil authority
8. Christian liberty
9. The problem of offense or scandal
These topics become one of the organizational structure by which Calvin organizes his Institutes in the 1539 edition onward (the original 1536 edition follows a catechism method adapted from Luther's Small Catechism). Thus, following Paul, Calvin and other Reformers used these commonplaces as interpretive keys to unlocking the major teachings of the Bible. They would organize the content, or testimony of Scripture under these heads in order the the preachers and teachers of the Church would know the overarching theological themes of Scripture.
It is also in Muller that we find Calvin's pedagogical division of labor. He did not attempt to write a comprehensive exposition of Scripture in his Institutes. Rather, in the interests of brevity, he sought to use the Institutes as a treatment of the common topics with some measure of disputation, but without a lengthy exegesis of Scripture. The Commentaries would be where Calvin would avoid delving into the disputed passages or common topics of doctrine, but would follow closely the thought of the Biblical writer, only occasionally delving into doctrinal extensions or disputations. In the sermons Calvin would depart from his attempts at brevity and engage in lengthy amplifications of his text in the form of direct applications to the congregation. He generally avoided disputation, and where he avoided numerous cross-referencing in the Commentaries, he indulged in them heavily in the sermons.
An exception to the division of labor occurs in Blacketer's analysis of Calvin's Harmony of Moses. Here Calvin arranges the materials of the last four books of the Pentateuch into an order more conducive to both doctrinal exposition and chronological narrative. The interpretive key for the harmony is the Decalogue, or Ten Commandments, which Calvin considers to be Scripture's own commonplaces for the doctrines and laws set forth in the Pentateuch. The Decalogue is a summary statement of God's Law from which all the ceremonial and civil laws of the Pentateuch are derived or deduced. Calvin goes to great lengths to categorize the laws according to their ceremonial or civic (or both) nature, commenting on the extent to which they are abrogated in the work of Christ, or by the limits of the historical context of Ancient Israel.
Blacketer points out that Calvin's Commentary in the Harmony does not follow the proposed brevity that Calvin claimed for himself. Blacketer's argument is that Calvin did not intend to use the doctrinal insights for a new edition of the Institutes, and so he saw fit to include them within the Commentary itself, despite having chastised Bucer for doing that very thing. Although this explanation seems plausible, it may also be necessary to consider the nature of the Harmony as such. Calvin is not merely attempting to follow the thought of Moses, but to improve upon its organization for the purposes of pedagogy for the Church. In such a case, Calvin is working both as Commentator AND as Expositor of the fundamental doctrines set forth by the text (i.e., the function of the Institutes). Calvin's Harmony may be more than a harmonizing the Pentateuch--it may also be harmonizing Commentary with the System of doctrine set forth in the commonplaces.
In any case, what is quite clear from Muller and Blacketer's scholarship is that the modern divide between systematic and biblical theology is foreign to the sixteenth century context. Such bifurcations, if there were even hints of them, are not matters of strict division of content, but rather a conscious division of labor suited to the pedagogical ease and needs of unlearned pastors and teachers in the Church. Calvin did not derive his dogmatics apart from his exegetical work in preparing commentaries and sermons for his congregation. Indeed, it was only because Calvin was constantly studying the Scriptures to exposit them for the congregation and for the future pastors and teachers that Calvin arrived at his dogmatic conclusions.
Finally, and the most important application to be taken, we should recognize that reading Scripture is of very little profit apart from understanding the doctrine that the Scripture displays. A corollary to understanding doctrine is that there must be a proper organization, method, or order of thought by which we can discern what Scripture teaches. For Calvin, the right order of teaching came via the classical rhetorical idea of commonplaces, which he saw applied by the Holy Spirit through Paul's ordering of the epistle to the Romans, and through Moses's development of the Law by means of the Ten Commandments. We should remember that rhetoric, like logic, though it was developed by the Ancient Greeks, is not inimical to Scripture, but is rather revealed in its pages to be the very means by which the Holy Spirit has chosen to reveal His thought to us. In that sense we may be humbled by God's choice to enlighten pagan minds to teach His holy saints about Himself. We may, like Augustine says, plunder the riches of Egypt to our profit, and to the glory of God.