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‘Not a Cistern, but a Fountain’

The God who is Perfect, Pure, and Simple   

Tertullian’s famous quip, “What hath Athens to do with Jerusalem?” is misleading. While he, like the apostle Paul before him, was right to raise concern over certain vain philosophies perpetuated by the Greeks, even he had to acknowledge the unavoidable task of interpreting the fundamental nature of reality. This quest for understanding has captivated the minds of philosophers since Thales first raised questions concerning the nature of being. The desire to speak honestly concerning the things of the earth culminates in the central question: what does it mean to be? The answer to this question undoubtedly bears on all things which are, which raises the stakes on incorrect conclusions. If one rightly understands reality, his metaphysical convictions will serve as a guide on the path of truth. If one misunderstands reality, however, these incorrect metaphysical convictions will lead him off the path of reality and into chaos.

The question of being was not lost on Moses and the Israelites. In Exodus 3:14, God revealed himself to Moses by saying, “I am who I am.” As the one who simply is being itself, God identified Himself to Israel as the self-sufficient creator and sustainer of all that is becoming. While Moses did not flesh out all the metaphysical implications at the burning bush, the seeds of Christian metaphysics were indeed sown on that holy ground.

As the one who simply is being itself, God identified Himself to Israel as the self-sufficient creator and sustainer of all that is becoming. Click To Tweet John Kenney points out, “God is, in consequence, unbegotten, unchangeable, and incorruptible. This fusion of the Platonic conception of being itself with the God of Exodus 3:14, who identified himself as the one who is, will henceforth become the linchpin of Christian metaphysics.”[1] As the ensuing chapters of Exodus demonstrate, Egypt’s gods were no match for the eternal and unchanging God who does not derive His existence from another.

The seeds of classical Christian metaphysics that God planted in the desert were watered by the New Testament authors and further cultivated by the Fathers, Medieval Scholastics, and Reformed Orthodox, resulting in a set of commonly held assumptions about the nature of reality. More importantly, these shared assumptions resulted in the confession of common doctrinal truths concerning the nature of God. At the fount of this stream was the concept of God as actus purus, or pure act.

Pure Act and Perfection

Actus purus asserts that God is perfect and lacks nothing. Since God is pure actuality, there is no passive potentiality within God that is waiting to be actualized. As actus purus, God is fully in act and is thus eternally actuating his active potential. Put simply, God’s pure actuality guarantees his eternal perfection and self-satisfaction.

The seeds of classical Christian metaphysics that God planted in the desert were watered by the New Testament authors and further cultivated by the Fathers, Medieval Scholastics, and Reformed Orthodox. Click To Tweet In arguing his five ways for proving God’s existence, Thomas Aquinas demonstrates that everything currently in motion must be actualized by something other than itself.[2] Nothing can be in motion without prior actualization because objects cannot actualize themselves. Based on this reasoning, Thomas makes a critical move. Though temporally speaking, potentiality is prior to actuality, logically speaking, actuality must precede potentiality because nothing can actualize itself. Thus, for all actualized movers to exist, behind them must be an unactualized first mover. Thomas concludes that the first mover “must be pure act, without admixture of any potentiality…Everything which is in any way changed, is in some way in potentiality. Hence it is evident that it is impossible for God to be in any way changeable.”[3] Herein lies the metaphysical connection between God’s pure actuality and the fullness, or perfection, of His divine attributes.

Pure Act and Simplicity

Aquinas sees this perfection in relation to the simplicity of God, meaning that God is not composed of parts. According to Aquinas, God is not a composite being made up of potentiality and actuality but rather is pure actuality itself. This means that God is not a being that changes or becomes something else, but rather, God is the ground of all being and the source of all change. Aquinas further demonstrates that all other beings are composed of potentiality and actuality. Humans, he explains, have the potential to become more virtuous or more knowledgeable, but we are not purely actual beings like God.

Etienne Gilson also demonstrates the connection between pure actuality and divine simplicity. He writes, “To say that God is absolutely simple, since He is the pure act of existing, is not to have a concept of such an act, but to deny Him… any composition whatsoever.”[4] As Moses learned in the desert, God simply is. Simplicity can be construed negatively and positively. Negatively, God is not composed of parts. Positively, God is pure existence.

Ed Feser explains, “Pure active potency or power unmixed with any passive potency or potentiality is just pure actuality and identified by the Scholastics with God; in everything other than God active potency is mixed with passive potency.”[5] God is pure being, and thus, He is never becoming, which is why the Scholastics found no trouble equating the God of the philosophers with the Triune God of the Scriptures.

Negatively, God is never in a state of becoming because any change necessitates that God is either changing for the better or for the worse. Both options prove perilous for upholding the absolute perfection of the God of the Bible. Positively, God is pure actuality, always being and never becoming. Thus, all of God’s metaphysical attributes come together to affirm that God is the one, simple, eternal, infinite, immutable, omniscient, omnipresent, omnipotent, first cause of all that exists who has life in himself.

Theological Implications of Pure Act

The concept of actus purus has significant implications for our understanding of God’s nature and existence. It suggests that God is not a being among beings but the source of all being. As pure act, God is not subject to the limitations of created beings, such as change or imperfection. God is not a being that exists within the world but is, instead, the ground of all being and existence. He is, therefore, self-existent and a se. Simplicity can be construed negatively and positively. Negatively, God is not composed of parts. Positively, God is pure existence. Click To Tweet

Far from being a revisionist position, the above articulation of actus purus is a core metaphysical assumption of the conciliar Christian Tradition. Carl Trueman makes this point clear by writing, “Indeed the notion of God as simple, as Him being pure act, and as thus being perfect in terms of having no potentiality, is an obvious point of continuity between Thomas and the Reformed.”[6] Peter van Mastricht succinctly affirms the essential link between the metaphysics of actus purus and God’s simple essence, stating, “All the attributes together in God are nothing but one certain most simple and most pure act, his very essence, and his infinite perfection.”[7] Similarly, Stephen Charnock speaks of God as “nothing but vigor and act,”[8] and John Owen affirms that God is “always actually in being, existence, and intent operation.”[9] Richard Muller sums up the post-Reformation consensus:

God, of course, is actus purissimus, pure or absolute actuality, and is, therefore, in no way potential: there is not potency in God for God to become other than what he is inasmuch as there is no actuality beyond God capable of changing God or of drawing God toward itself. God, in short, is perfect and immutable.[10]

The above consensus affirms the essential dogmatic link between pure actuality and God’s simple and immutable perfection.

Denying Pure Act

Though actus purus was a lynchpin in premodern theology, a glance at many 20th century theological texts reveals that pure actuality has not only fallen out of favor but is hardly considered worth mentioning at all. While premodern treatments of God’s nature could refer to actus purus in a passing manner, this cannot be done today. Where classical Christian metaphysics was assumed, actus purus was assumed, but in a post-Kantian world where metaphysics is outright denied or ignored, so is actus purus.

James Dolezal demonstrates how denying the metaphysics of pure actuality leads to theological revisionism. He writes,

After Hume and Kant’s attack on the perennial Aristotelian philosophy, many a Christian theologian opted to abandon, rather than defend, the metaphysical structure (regarding being, becoming, and causation) in terms of which simplicity had been so meticulously developed. Indeed, many Christian theologians and ministers retreated from the field of metaphysics altogether and retrenched themselves in their Bibles, assuming that the Bible’s teaching could be successfully preserved without committing oneself to a particular understanding of being.[11]

Thus, all of God's metaphysical attributes come together to affirm that God is the one, simple, eternal, infinite, immutable, omniscient, omnipresent, omnipotent, first cause of all that exists who has life in himself. Click To Tweet While many of these attempts to read the Bible without certain metaphysical commitments were genuine efforts to preserve the faith once handed down to the saints, the resulting theological revisionism significantly altered vital doctrines of the Christian faith to make them compatible with the post-Kantian metaphysics of modernity. In responding to a denial of divine simplicity based on a modern, mutable Christology, Dolezal notes that the modern aversion to simplicity is really an aversion to immutability and pure actuality. He defends the classical notion of divine simplicity by noting that it is rooted, “in pure actuality, and pure act will not allow the mutation that is claimed [by modern, mutable Christology] to be requisite for the Word to become flesh.”[12] God’s immutability as actus purus has gone from a starting point to an afterthought.


James writes, “Every good gift and every perfect gift is from above, coming down from the Father of lights” (1:17). Because God is actus purus, Christians can be assured that He never acts to gain, for He requires nothing. Instead, as He is simple perfection, He always acts to give. His gifts are pure grace!

The implications of actus purus do not end with divine simplicity but touch nearly every other area of Christian doctrine. For example, Stephen Charnock considers the beatific vision while demonstrating just how vigorous and lively the purely actual, immutable, and simple God truly is. He writes:

God…sitting upon his throne of grace, and acting according to his covenant, is like a jasper-stone, which is of a green color, a color always vigorous and flourishing; a pure act of life, sparkling new and fresh rays of life and light to the creature, flourishing with a perpetual spring, and contenting the most capacious desire; forming your interest, pleasure, and satisfaction; with an infinite variety, without any change or succession; he will have variety to increase delights, and eternity to perpetuate them; this will be the fruit of the enjoyment of an infinite and eternal God; he is not a cistern, but a fountain, wherein water is always living, and never putrefies.[13]

May the Church recover this classical confession of the Triune God who is perfect, pure, and simple.


[1] John Peter Kenney, “Platonism and Christianity in Late Antiquity,” in Christian Platonism: A History, ed. Alexander J. B. Hampton and John Peter Kenney (Cambridge, United Kingdom: Cambridge University Press, 2021). 171.

[2] St. Thomas Aquinas, Summa Theologiae: Volume 1, Christian Theology: 1a. 1, ed. Thomas Gilby (Cambridge: Cambridge University Press, 2006). ST,Ia9.1. See also ST, Ia.2.3; SCG I, c. 17.

[3] Ibid.

[4] Étienne Gilson, The Christian Philosophy of St. Thomas Aquinas, trans. L. K. Shook (Notre Dame, IN: University of Notre Dame Press, 1994), 97.

[5] Edward Feser, Scholastic Metaphysics: A Contemporary Introduction (Heusenstamm: Editiones Scholasticae, 2014), 39.

[6] Carl R. Trueman, “The Reception of Thomas Aquinas in Seventeenth-Century Reformed Orthodoxy and Anglicanism,” in The Oxford Handbook of the Reception of Aquinas, ed. Matthew Levering and Marcus Plested (Oxford: Oxford University Press, 2021), 212.

[7] Petrus Van Mastricht, Theoretical-Practical Theology, Volume 2: Faith in the Triune God, ed. Joel R. Beeke (Grand Rapids, Michigan: Reformation Heritage Books, 2021), 117.

[8] Stephen Charnock, The Existence and Attributes of God, ed. Mark Jones, Updated and Unabridged (Wheaton, IL: Crossway, 2022), 1:289.

[9] John Owen, The Works of John Owen (Carlisle, PA: Banner of Truth, 1968), 12:71.

[10] Richard A. Muller, Post-Reformation Reformed Dogmatics, Vol. 3: The Divine Essence and Attributes, 2nd ed. edition (Grand Rapids, Mich: Baker Academic, 2003), 529.

[11] James E. Dolezal, All That Is in God: Evangelical Theology and the Challenge of Classical Christian Theism (Grand Rapids, Michigan: Reformation Heritage Books, 2017).

[12] Dolezal, “Neither Subtraction, Nor Addition.” 134.

[13] Charnock, The Existence and Attributes of God (2-Volume Set), 1:299.

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Lance English

Lance English is an editor for Credo Magazine and a Ph.D. student in Systematic Theology at Midwestern Baptist Theological Seminary. He serves as a pastor at Trinity Church in Kansas City and is married to Brielle.

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