The simplicity of God means that the being of God is "simple" and beyond composition. God is not complex in the sense that he is made up of distinct properties or constituents, nor can it be said that the Father, Son, and Holy Spirit comprise three parts of God. Also called divine simplicity, it forms a significant corollary of both the unity and trinity of God.
The doctrine of divine simplicity is associated with the classical theism of Augustine, Anselm, Aquinas and their adherents, whereby God is understood as radically unlike creatures in that he is devoid of any complexity or composition, whether physical or metaphysical. Some modern critics, such as Alvin Plantinga,  challenge the doctrine as being incoherent. 
" Emil Brunner, for example, comes down especially hard on the entire concept, criticizing the general trend of the Protestant Scholastics to ground their entire systematic theological formulations in this idea of simplicitas Dei, and arguing that the notion only arises 'if we make the abstract idea of the Absolute the starting-point for our thought. This is simply the undifferentiated Monas of Neo-Platonism modified by Theism.' Again: 'Here [in speaking of simplicity], we are not dealing with an Attribute of God at all, but with the fact that the idea of the Absolute permits no differentiations.' " 
- God without Parts: The Doctrine of Divine Simplicity, by James Dolezal
- Augustine, The Trinity . New City Press, 1991. bks 5–7
- Stephen R. Holmes, "' Something Much Too Plain to Say': Towards a Defence of the Doctrine of Divine Simplicity.” In Listening to the Past: The Place of Tradition in Theology , 50–67. Baker Academic, 2002.
- Andrew Radde-Gallwitz, Basil of Caesarea, Gregory of Nyssa, and the Transformation of Divine Simplicity . Oxford University Press, 2009.
- Richard A. Muller, Post-Reformation Reformed Dogmatics: Volume Three: The Divine Essence and Attributes. Baker Academic, 2003. esp. pp. 38–67; 70–76; 205–215; 271–297.
- Lewis Ayres, Nicaea and its Legacy: An Approach to Fourth-Century Trinitarian Theology . Oxford University Press, 2004. esp. chs. 11, 14, and 15.
- Karl Barth, _Church Dogmatics_II/1, pp. 443–461. Cf. _CD_II/1, pp. 260ff., esp. 327–331.
- James Dolezal, God Without Parts: Divine Simplicity and the Metaphysics of God's Absoluteness . Wipf & Stock, 2011.
- Christopher A. Franks, "The Simplicity of the Living God: Aquinas, Barth, and Some Philosophers" Modern Theology 21:2 (2005): 275 –300.
- ↑ Alvin Plantinga, Does God have a Nature?(Milwaukee: Marquette University Press, 1980).
- ↑See Divine Simplicity, Stanford Encyclopedia of Philosophy.
- ↑Jules Grisham, Euthyphro, God's Nature, and the Question of Divine Attributes, Part 1.
- "Euthyphro, God's Nature, and the Question of Divine Attributes" ( Part 1, Part 2, & Part 3), by Jules Grisham
- Divine Simplicity, Stanford Encyclopedia of Philosophy
- Divine Simplicity: A Bibliography(an atheist's compilation)
- Making Sense of Divine Simplicity(PDF), Jeffrey E. Brower, Purdue University.
- St. Thomas Aquinas: The Doctrine of Divine Simplicity, Michael Sudduth, Analytic Philosophy of Religion
- On Alvin Plantinga's Notion of the Divine Nature, by Scott David Foutz
- Properties, Conflation, and Attribution: the Monologion and Divine Simplicity, by Siobhan Nash-Marshall
- A Theological and Philosophical Defense of Divine Simplicity Against the Criticisms of William Lane Craig, by Matthew Graham