The attribute of God’s eternality is probably one which is easily confessed. In some ways we seem to have some better concept of that, as opposed to something like immutability or impassibility. God is the King eternal (1 Tim 1:17). He is the Alpha and Omega (Rev. 1:8). He is the One whose years have no end (Ps 102:27). He is from everlasting to everlasting (Ps 90:2). We sort of understand time. Our days are ordered by it. We wear watches that measure it. We teach our kids to tell time. It’s simple. That is, until we actually start to sit down and think about time. What is time? How do we explain time? What does it mean if we think of God existing outside of time? Our brains begin to overload and if we’re not careful we’ll blow a gasket. I remember reading Augustine’s Confessions for the first time and coming to his sections on time and thinking, “This is amazing. I’ve never thought about time like this. It makes so much sense. I wish I understood what he was saying.” God’s eternality is like that. It is easy to confess and yet difficult to understand.
What do we mean when we say that God is eternal? The basic idea is that God does not experience successive states of being and thus has no future and no past. God does not exist in time, as if time is something that is over him. Francis Turretin wrote, “True eternity has been defined by the Scholastics to be ‘the interminable possession of life – complete, perfect, and at once.’ Thus, it excludes succession no less than end and ought to be conceived as a standing, but not a flowing, now.”  What Turretin means by this is that God exists in an all-encompassing now. There is no movement from past to future through the present. Everything for God is the same and immutable (unchangeable). Aquinas would argue that God is “an instantaneous whole lacking successiveness.”
While we confess that God is everlasting, his eternality is more than just everlasting. Some theologians and philosophers will use language like this. They will affirm that God is everlasting (having no beginning or end) but not necessarily affirm that God is eternal (without successiveness). By this they mean that God is without beginning or end, but that God has entered into time and exists in time. It is claimed that this is necessary for God to be a Creator or for God to have a relationship with man. Nicholas Wolterstorff, William Lane Craig, John Frame, and K. Scott Oliphint all hold to various forms of this idea. The common denominator being that God is in some sense temporal. But there are grave issues with this.
If God is temporal, then God is bound by something outside of him. He is measured by time. This would violate God’s infinity. If God is temporal, then it would also require God to give up his simplicity. The simplicity of God states that God has no parts or components. He simply is. But if God is temporal, then that gives God potential. Potential in humans is usually a good thing. Your kid’s coach might say, “He’s got a lot of potential.” And you would be encouraged. But a God with potential means that he is not currently all that he could be. But a God subject to successiveness means that the God who was is not necessarily the God who will be. This introduces change into the character of God. Can you trust a God who changes? Will he change for the better or the worse?
God’s eternality, therefore, has three marks. First, it has no beginning. There was never a time when God was not. Second, it has no end. It is true that we sometimes apply the term “eternal” to some things which possess only this mark, e.g. angels or a human’s soul. Third, it has no successive moments. God is an “instantaneous whole.”
The attribute of eternity is crucial to the character of God. Dolezal argues, “Nothing that begins to exist can be regarded as properly divine.” We cannot limit God with our notions of time. And we need to understand that when Scripture speaks of God in ways that seem to bind him in time, it is anthropomorphic language. These are examples of God condescending to our level that we might understand. We must approach these issues with a proper humility toward God’s perfections. When Augustine was asked sophistic questions like, “What was God doing before he created everything?” he would answer, “Making up Hell for people who ask stupid questions like that.” The Scriptures are clear that God is eternal and unchangeable. His perfect is always perfect or else it wouldn’t be perfect. There is no progression or becoming in God. There is no was or will be in God. God is the great “I AM.”
 Turrettini, Giger, and Dennison, Institutes of Elenctic Theology. Vol. 1, III.10.6.
 See Dolezal, All That Is in God, 89–97.
 Dolezal, 97.
 Augustine, The Confessions and Letters of St. Augustine, XI.14.