Equity makes its appearance in theological and confessional treatments as a way of understanding the place of Old Testament law in the New Testament era. It is invoked e.g. in the Westminster Confession of Faith as such an interpretive principle. A good treatment of the theological usage of equity can be consulted here, but, there as typically, the Roman-legal background of the concept is only hinted at, if mentioned at all. To fill this void I offer the following considerations:

1. Sohm’s historical outline shows how Roman law provided the basis for the doctrine of equity, upon which all further development proceeded. The Romans developed the idea of universally valid legal principles, by a process of inference and comparison at the level of court practice. Equity developed in tandem with the notion of a universal law of nations — ius gentium or ius commune.
2. Stahl describes the achievement of the Romans as having discovered the principle of “the nature of the case,” i.e., the discovery of a framework for classifying the various cases and delineating the characteristics (Private Law, pp. 194ff.). But this also points up the shortcomings of the Roman achievement. It is restricted to practice and insufficiently informed by higher principles. Donald Kelley (The Human Measure: Social Thought in the Western Legal Tradition, ch. 9) describes how canon law developed a more Christianized concept of equity. In fact, he argues that canon law functioned as a sort of equity overarching the particular legal systems of the West. Berman (Law and Revolution, ch. 6) points up the many ways that canon law influenced secular legislation in this manner.
3. The concept of equity also became attached to Roman law generally, in the form of Ius Commune (common law). Sohm already hinted at this functionality when he showed how equity developed together with the concept of ius gentium (law of nations), a common overarching law (my book A Common Law also discusses this). This understanding of the role of Roman law in the West was developed most famously by one of the premier legal historians of the last century, Francesco Calasso. Hitherto scholars viewed the Roman law as a kind of residual positive law, valid directly in the absence of relevant local or national law. But Calasso showed that it functioned as a form of equity in the lawmaking and interpreting process. Manlio Bellomo was a disciple of Calasso’s and put forward the thesis in The Common Legal Past of Europe 1000-1800, which I highly recommend.
4. England developed a separate jurisdiction of equity alongside the common law courts, in the Court of Chancery. One good place to read about this is C.K. Allen’s Law in the Making, pp. 382ff. Of course, the Court of Chancery and its dilatory working was the target of Dickens’ venom in Bleak House.
5. Stahl cautioned against the application of equity in adjudication. He argued that equity cannot be used as a criterion for invalidating standing law. It can only be used where law is unsettled or where it is specifically attributed a role in the legal system. “The essence of justice is to maintain a given order, law and right inviolably intact, while the essence of equity is precisely disregarding all prior existing order, all prior conferred law and right, simply to restore the equality of advantages and disadvantages (aequum) among those concerned…. In terms of its deeper root, equity rests partly on the original equal entitlement of men and partly on the equal love for all, which yields an equal standard everywhere where no special law, right, or right-founding act effects a preference. In this root, equity is reunited with justice, it consisting merely in disregarding those intervening grounds. A decision in accordance with equity is therefore in order where such facts grounding a specific inviolable right are truly lacking, e.g., in the settling of confused boundaries, while in the contrary case it is a censurable violation of justice” (Principles of Law, pp. 126ff.).
6. As an example of jurisprudential treatment of the concept in the 17^th century, Althusius wrote “Æquitas contra stricto juri opposita” [equity is contrary to strict justice] and defined it as a principle for interpreting standing law (discussed in the chapter “De naturali juris interpretatio” or “on interpretation of/from natural law).  See the Dicaelogicae, pp. 48f.

All of these Roman-legal considerations in one way or another informed the concept of equity as understood in the mid-17^th century, and should be taken into consideration as background for the theological discussions and treatments invoking the topic, especially in view of the fact that many leading theologians and intellectuals were trained in Roman law (e.g., Calvin, Beza, Hotman, De Mornay, Althusius, Rutherford).