When the apostle Paul visited Athens and was confronted by the scope and depths of idolatry there, he said, “I see that you are very religious…” and went on to alert them to the existence of one God they did not chose to know. Of course, he didn’t stop there; instead he proceeded to describe the God they did not know: “The God who made the world and everything in it, being Lord of heaven and earth, does not live in temples made by man, nor is he served by human hands, as though he needed anything, since he himself gives to all mankind life and breath and everything” (Acts 17:24-25). This statement is a classic introduction to the Christian understanding of the existence of God.
Paul immediately pointed them to the fundamental ontological distinction between the creator and the creation; between the God “who made” and “everything” that was made. On the one hand there is God the Maker and on the other hand there is everything that is not God. He was teaching them and us that we cannot and must not speak of God and speak of creatures as if they belong to the same category of “being,” as if he were simply a larger version of ourselves. The apostle then unpacks this in two directions:
First, God the Creator is the Lord of heaven and earth; he is not confined within the time and space of his creation, rather, he exists in every reality both inside and outside created reality. Had Paul expounded Genesis 1 he would have made the point that there is nothing prior to God’s act of creation and no attempt to give an account of God’s own beginning. Had it done so, God would have ceased to be God. All reality, visible and invisible, material and spiritual exists by divine faith: “He spoke, and then it was, he commanded, and then it stood!” (Ps. 33:8-9). God alone is the source of all that exists, and all that is exists to serve him as Lord.
Second, God is independent of his creation, God is not “served by human hands” because he doesn’t need anything, that is to say he is entirely self-sufficient and does not depend on anything outside of himself for his existence or blessedness.
Third, God is the superabundant source of the creature’s total existence (“he himself gives to all mankind life and breath and everything”); he is, in himself the fullness of life, being and everything. Anselm of Canterbury wrote, “You alone …Lord are what you are and are who you are…He alone has of himself all that he has, while other things have nothing of themselves. And other things, having nothing of themselves, have their only reality from him.” 
This turns us to consider that perfection within God we call aseity. The word aseity comes from the Latin “a” (from) and “se” (self) meaning that God exists “in and of’ himself,” or “from himself.” Herman Bavinck writes: “God is exclusively from himself, not in the sense of being self-caused but being from eternity to eternity who he is, being not becoming. God is the absolute being, the fullness of being, and therefore always eternally and absolutely independent in his existence, in his perfections, in all his works, the first and the last, the sole cause and final goal of all things.” 
One corollary of this is to say that God is who he is. As Thomas said, his essence is his existence. This means God is his attributes – they are not accessories, they are not accidental to who he is, rather he is his attributes: God “is” Holy, Love, Spirit, Wisdom, Power and he exists like this as Father, Son and Holy Spirit.
The idea of aseity stems from the self-naming of God recorded in Exodus 3 where Moses asked God to identify himself and received the answer: “I AM THAT (or WHO) I AM.” This self-naming reveals that God is not contingent on anything or anyone outside of himself; he does not need us to give him anything. He is free from all coercion and constraint, free to act according to his own will and in a manner of his own choosing.
So, the apostle Paul says, he has mercy on whom he will have mercy and hardens whomsoever he pleases (Rom. 9:7-13). The apostle is responding to all who object to such total freedom: “does not the potter have authority over the clay to make from the same batch a vessel of honor or a vessel for dishonor?” (Rom. 9:19-24). In utter freedom God created the world – he did this not because he lacked anything or needed anything, but because he freely chose to do so. He was entirely free to choose to create or not create as he pleased. God’s existence is absolute and underived. He did not cause himself, nor did he create the universe out of any form of necessity, as if he were diminished without it or enhanced with it. He created the world simply because he was pleased to do so. As Augustine said, “God exists in the supreme sense, and the original sense, of the word. He is altogether unchangeable, and it is he who could say with full authority ‘I am who I am.’”  This self-naming reveals that God is not contingent on anything or anyone outside of himself; he does not need us to give him anything. He is free from all coercion and constraint, free to act according to his own will. Click To Tweet
Aseity ultimately is an affirmation of the essential beauty, goodness, fullness and generosity of God; the perfection and completeness of his being in himself. In Acts 17, God freely “gives life and breath and everything” to everything and everyone, everywhere. He gives existence to everything that exists. “In him we live and move and have our being (existence).” The divine name, “I AM WHO I AM” reveals God to be rich and full of existence, power, and glory.
The divine name indicates his perfection and plenitude as Father, Son and Holy Spirit. To observe the multiplicity and redundant diversity of all that he has made in the created world of nature gives us only the faintest glimpse of the riches of life that resides in God himself considered apart from this created reality of which we are a part. It also presents us with the tantalizing prospect of the “riches of his glorious inheritance in the saints…that you may be filled with all the fullness of God” (Eph. 1:18; 3:19). To contemplate the inner life of the Holy Trinity, as revealed in Holy Scripture, is to see a timeless state of beatitude and fullness of life: “from his fullness have all we received” (Jn. 1:16).
Here we think about the fact that God is immutable but not static; God is impassible but not passive; God is immoveable but not inactive. John Webster asserts that “God is from himself, and from himself God gives himself.” He goes on to add: “Aseity is life: God’s life from and therefore in himself.”  Aseity is the timeless and lively perfection of the Father, Son and Holy Spirit. He expresses himself in creation; in humanity made in his image; in the prophets through whom he has spoken; and supremely, in Christ who, incarnate, is “the image of the invisible God.”
To understand this we need to remember that he has revealed himself to be the Holy Trinity, and, within the triune life, to be marked by personal relations of paternity, filiation and spiration. In timeless eternity the Father generated (or begot) the Son without any beginning; in timeless eternity the Son is generated or begotten of the Father without any beginning; and in timeless eternity the Holy Spirit was spirated and as such proceeded from the Father and the Son without any beginning. In Jesus’ words,”the Father has life in himself” and has “given the Son to have life in himself” (Jn. 5: 26). Similarly, the Holy Spirit is both generative of natural life and regenerative of spiritual life. We find him breathing life into creatures to make them live at creation and, in recreation he breathes new life into God’s elect. Therefore God the Holy Trinity is the living God.
All this begetting, being begotten, and proceeding, points to the eternal life and movement that is intrinsic to God’s eternal nature as God. To have life in himself means that God lives in a way that is essentially active and self-giving. So, the Father loves and glorifies the Son as the Son loves and glorifies the Father in the Holy Spirit and as Father, Son and Spirit inseparably love and will glorify the elect. Of course, we have to add caveats here: God’s motion entails no expenditure of power on his part, nor does it require any change within him, it adds nothing to nor subtracts anything from him. Click To Tweet
This challenges notions of God as the “unmoved mover” who is passive in himself. The God of Holy Scripture is never conceived of as being in potency (that is, of eternally having the potential to act but never acting until he begins to create). Rather, as the Holy Trinity he is always “in act,” and that action has no beginning and no end; it is timeless and eternal. Within his triune life he is always begetting and proceeding, loving and blessing. God’s perfection is not “mere immobility, rest conceived as simple absence of motion. To say God is ‘a se’ is to say God ‘lives’ a se. God is, and therefore God lives, and therefore God moves.” 
Thus it is that God who is of himself, from himself who also gives himself in creation, incarnation and re-creation. He who has life in himself delights to give life to others. It is therefore not necessary or safe to posit that God changes at creation or in redemption by acquiring new attributes or acting differently from what he eternally is. The God, who timelessly generates the Son and breathes forth the Spirit, chooses to create the universe and living creatures. Of course there is an ontological difference between these two actions: the one eternal the other temporal, the one infinite the other finite, the one invisible the other visible. The Son’s begetting and the spiration of the Spirit belongs to the nature of God in himself as God. The making of the universe belongs to the will and word of the triune God.
Divine aseity is not a part of God or an aspect of God’s nature or character any more than love, wisdom, power or justice are. There is nothing that lies behind God but God! He is who he is, for his essence is his existence. All things are from him, through him, and to him. He is the all-sufficient explanation of his own reality, his own essence and existence.  God is not passive even though he is impassible; he is fully in act, fully present in the economy, fully sustaining all things visible and invisible, fully committed to those whom he loved since before the foundation of the world. He has all life, glory and blessedness in and of himself in the perfection of his own nature, yet he grants us eternal life by his gracious turn towards us. We can testify that “For his pleasure all things are and were created.” Yet nothing he created adds value to him: “Can a man be profitable to God? Surely he who is wise is profitable to himself. Is it any pleasure to the Almighty if you are in the right, or is it gain to him if you make your ways blameless?” (Rev. 4:11; Job 22:2-3).
 Quoted in John Webster, God without Measure: Working Papers in Christian Theology, Volume 1: God and the Works of God (Edinburgh: T&T Clark, 2015), p.15.
 Herman Bavinck, Reformed Dogmatics, Volume 2: God and Creation (Grand Rapids: Baker Academic, 2004), p.152.
 Augustine, On Christian Teaching (Oxford: Oxford University Press, 1997), I.xxxii.
 Webster, God without Measure, p. 19.
 John Webster, “God’s Perfect Life” quoted in Stephen J. Duby, Divine Simplicity: A Dogmatic Account (Edinburgh: T&T Clark, 2015), p.119.
 see James E. Dolezal, God without Parts: Divine Simplicity and the Metaphysics of God’s Absoluteness (Eugene: Pickwick, 2011), p.71.