The nature of God is paramount in apologetics, so today we will begin to explore what that nature entails. The article below explains the aseity of God.
Aseity (Self-Existence). Most classical theists see God’s Aseity or Pure Existence as a key attribute. The early Church Fathers, as well as Augustine (354–430), Anselm (1033–1109), and Aquinas, continually cite the Bible in support of this position. In defending God’s self-existence (aseity) classical theists such as Aquinas are fond of citing Exodus 3:14 where God identifies himself to Moses as “I Am that I Am.” This they understand to refer to God as Pure Being or Existence.
God is Pure Actuality, with no potentiality in his being whatsoever. Whatever has potentiality (potency) needs to be actualized or effected by another. And since God is the ultimate Cause, there is nothing beyond him to actualize any potential (i.e., ability) he may have. Nor can God actualize his own potential to exist, since this would mean he caused his own existence. But a self-caused being is impossible, since it cannot create itself. Something has to exist before it can do anything. Even God cannot lift himself into being by his own ontological bootstraps. Thus, God must be Pure Actuality in his Being.
Of course, God has the potential to create other things. But he cannot bring himself into being. He always was. And while God has the potential to do other things, he cannot be anything other than what he is. He has the power to create other things (active potency), but he does not have the power (passive potency) to exist in any other way than he does, namely, as an infinite, eternal, necessary, and simple Being.
God’s aseity means that he is Being; everything else merely has being. God is Pure Actuality; all other things have both actuality and potentiality. Thus, God cannot not exist. All creatures can be nonexistent. That is, they have the potentiality for nonexistence. Only God is a Necessary Being. All other beings are contingent.
Geisler, N. L. (1999). In Baker encyclopedia of Christian apologetics (p. 282). Grand Rapids, MI: Baker Books.