Below is an excerpt from Chapter 2 of The Oxford Handbook of Reformed Theology , by Christopher Cleveland. Dr. Cleveland received his Ph.D. from the University of Aberdeen, where he studied under Professor John Webster. His research focuses on the relationship between medieval and Reformed thought. He is the author of Thomism in John Owen.

Reformed theology inherited several elements of theology from the Middle Ages. The use of medieval theology naturally varied according to each theologian, but there are several doctrines where medieval influence is visibly evident. In each case, the purpose is not to adopt the medieval system completely, but to utilize certain elements of it in the service of Reformed theology.
Existence and the Divine Name
Reformed theologians followed medieval thought in affirming that the name revealed to Moses upon Mount Sinai ‘I Am who I Am’ (Exodus 3:14) was the proper name of God, and that it revealed his self-existence and necessity. This understanding of the divine name has a long pedigree in medieval thought, building upon exegesis of the patristic era. Aquinas affirmed that it was the proper name of God (ST I, q.13, a.11, Resp.). Duns Scotus in his Tractatus de Primo Principio likewise noted that this name indicated God’s self-existence (Scotus, Tractatus de Primo Principio 1.2). This understanding of the divine name is nearly universal in Reformed thought. Calvin, for example, writes of this name, ‘This is very plain, that God attributes to himself alone divine glory, because he is self-existent and therefore eternal; and thus gives being and existence to every creature’ (Calvin 1979: 73–4). Francis Turretin explains that this name, Jehovah, reveals ‘the eternity and independence of God, inasmuch as he is a necessary being, and existing of himself, independent of any other, self-existent (autoōn)—“I am that I am” (Ex. 3:14)’ (Turretin 1992: i.184). However, unlike the medieval scholastics, Reformed theologians used linguistic and exegetical tools to examine the texts more closely in their original languages. Jerome Zanchi, for example, devotes nearly 28 folio columns to the discussion of this name in his De Natura Dei, both examining the Hebrew text and utilizing Thomistic thought in his theological formulation (Zanchi 1577: 39–52).
This name revealed that God possesses the power of existence in himself, in his own essence. All else is thus contingent, and is capable of non-existence. But God is necessary, as he alone truly exists, and possesses in his own essence the very power of existing. All else exists because of him, and in dependence upon him.
Simplicity
Connected with this understanding of the divine name is divine simplicity. Medieval theology, building upon patristic thought, saw the doctrine of divine simplicity arising from the revelation of the Tetragrammaton. This understanding of divine simplicity was developed to a great extent by Aquinas, who argued that as God is ‘Qui Est’, he is not composed of act and potency or matter and form as creatures are, but is rather pure act of being without any potentiality (ST I, q.2, a.3; ST I, q.3, a.2). This understanding sees God as the sole being who is without any potential to be greater or more perfect, but who possesses all perfection actualized. He is his attributes and his perfections.
Reformed theologians affirmed this doctrine strongly. John Owen writes:
God says of himself that his name is Ehejeh, and he is I AM,—that is, a simple being, existing in and of itself; and this is that which is intended by the simplicity of the nature of God, and his being a simple act. The Scripture tells us he is eternal, I AM, always the same, and so never what he was not ever.
(Owen 1965: xii.71)
Jerome Zanchi notes that this name reveals that God does not possess any quality or characteristic through another, but that his essence is his attributes, and that without composition (Zanchi 1577: 83). God cannot be other than himself or composed of parts in any way. Turretin notes that there is no composition of any kind in God, whether physical, logical, metaphysical, or between essence and existence (Turretin 1992: i.192).
The doctrine of divine simplicity became particularly relevant to Reformed orthodox theology as new heresies arose concerning the nature of God. Socinian theology in
particular posed a threat to the traditional doctrine of God, as it argued that God was corporeal, and composed of passive potency. In response, Reformed theologians such as John Owen used the doctrine of divine simplicity to argue that God is simple act, without any parts to be separated from His being, or anything prior to him (Owen 1965:xii.71). Likewise, Francis Turretin argued for divine simplicity against the anti-Trinitarian arguments of the Socinians, who argued that composition in the divine essence disproved the doctrine of the Trinity (Turretin 1992: i.191). Simplicity was essential to the Reformed defence of the orthodox doctrine of the Triune God, and thus played a key role in Reformed treatments of the doctrine.
Causality, Providence, and Predestination
The understanding of God as pure act of being without potentiality indicates that God exists in a relation of precedence and priority to all creaturely existence. As he is pure act, he can bring that which is merely potential into actual existence. This means that God is the First Cause of all creation, bringing all things into existence. This argumentation is first laid down in the opening section of the Summa Theologiae, and is utilized extensively throughout the rest of the Summa. Thomas uses it not only to express the way in which God’s existence is known and demonstrated in the world, (ST I, q.2, a.3) but also to demonstrate the necessity of grace for the creature to attain the end of salvation (ST I–II, q.109). Thus it is not only necessary that God be the unmoved mover with respect to creation. It is also necessary that God be the unmoved mover with respect to providence and predestination.
This Thomistic logic became very prominent in the de auxiliis controversy of the late sixteenth and early seventeenth centuries. When the Jesuit Luis de Molina developed the concept of scientia media, he was strongly opposed by Dominican theologians such as Domingo Banez and Diego Alvarez, who argued that the creature is incapable of action unless he is moved by divine physical premotion (Serry 1700; Eleutherius 1742; Bañez 1584; Molina 1588).
The Reformed utilized this same Thomistic logic in their debates with Arminian thought.
Every thing that is independent of any else in operation is purely active, and so consequently a god; for nothing but a divine will can be a pure act, possessing such a liberty by virtue of its own essence. Every created will must have a liberty by participation, which includeth such an imperfect potentiality as cannot be brought into act without some premotion (as I may so say) of a superior agent.
(Owen 1965: 119–20)
For John Owen, the nature of the created will is that it exists in reliance upon God, and cannot act in any way without the previous action of God moving it. Francis
Turretin notes that there is a previous and simultaneous concourse of divine action moving the creature (Turretin 1992: i.506–7). Petrus van Mastricht argues that it is
necessary for there to be a physical, not merely a moral or persuasive, premotion in the work of conversion (van Mastricht 1715: 660). Gisbertus Voetius notes that this premotion is the awakening of created power that is given to the creature in order that it may act (van Asselt et al. 2010: 151).
The purpose of the conception of physical premotion is to explain the manner in which God moves the creature by giving it life, power, and ability to act according to the divine will. The Reformed theologians of the seventeenth century found this Thomistic conception to be consistent with Reformed orthodoxy. It provided for them a precise explanation of the way in which God works all things according to the counsel of his will.
From The Oxford Handbook of Reformed Theology edited by Michael Allen and Scott R. Swain. Copyright © 2020 by Michael Allen and Scott R. Swain and published by Oxford University Press. All rights reserved.