In my studies of the Reformers and Reformed Scholasticism, two of the more practical and helpful teachings I found were the doctrines of the two kingdoms and natural law. In other terms, it was a great learning experience to see 1) how Reformed theologians typically distinguished between many aspects of the church and the civil government and 2) how they explained that God’s law is engraved on every conscience to some extent, which forms the regulations in the civil realm. I appreciate how David VanDrunen summarizes a few of these things after discussing the teachings of some key Reformed theologians.
“A first respect in which the natural law and two kingdoms doctrines were related in Reformed orthodoxy is that both natural law and the civil law were grounded in God’s work of creation and providence rather than in his work of redemption. As considered above, the Reformed orthodox writers perceived natural law to be given originally in the creational covenant of works and sustained along sinful human beings even apart from the redemptive covenant of grace. Similarly, they understood the civil kingdom to be established and governed by the triune God as creator but not by Christ in his specific role as mediator of redemption.
“This fundamental relationship between the Reformed orthodox natural law and two kingdoms doctrines is important background for the next point: natural law was considered the primary standard for the civil kingdom but not for the spiritual kingdom, where Scripture was primary. …In this, Reformed orthodoxy made a claim common to the Reformation and medieval traditions before it. That this grounding of civil law in natural law was closely related to and even incorporated into the two kingdoms doctrine is evident in [Francis] Turretin’s sevenfold distinction between civil and ecclesiastical power, in which he contrasts civil power as that which is regulated by ‘natural reason, civil laws, and human statutes’ and ecclesiastical power as that which is regulated by ‘the word of God alone.'”
“By such statements, Turretin and other Reformed orthodox writers did not mean to say that Scripture is irrelevant for the civil kingdom nor natural law for the spiritual kingdom. As discussed above, they drew instruction on civil concerns from the Mosaic law and Old Testament history. Nevertheless, they applied these things to the contemporary situation, at least in their more theoretically reflective moments, when they believed that they were rooted in natural law and therefore generally applicable rather than uniquely suited to the Mosaic theocracy. On the other side, they also believed that natural law has relevance for the spiritual kingdom, as illustrated in Westminster Confession of Faith 1.6….”
Based on my own readings and studies of the Reformation and Reformed orthodoxy, I believe that’s a good summary. You can find it on pages 208-9 of VanDrunen’s excellent book, Natural Law and the Two Kingdoms: A Study in the Development of Reformed Social Thought.