That he is self-existent; he has his being of himself, and has no dependence upon any other: the greatest and best man in the world must say, By the grace of God I am what I am; but God says absolutely-and it is more than any creature, man or angel, can say-I am that I am. Being self-existent, he cannot but be self-sufficient, and therefore all-sufficient, and the inexhaustible fountain of being and bliss.
Matthew Henry, Commentary on Exodus 3
It is not a particularly reformed distinctive to confess that God is eternal, all orthodox Christians through the passing of the ages have agreed that our God is eternal. It is one of the more explicit things about God that we find in the Scriptures. How we define that as compared to some, however, is where a few of the boundaries are set.
Some seek to define God’s eternality as simply an endless succession of moments; never-ending, and with no beginning. They say that God experiences time as we do, only that He experiences it in an infinitely greater and more innumerable way. This is not how classical Christian Theism, and the Reformed Confessions use the word eternal.
The best way that we can say what we know about God is to define what we deny about His existence. From this idea, words like immortal (not mortal,) immutable (not mutable,) invisible (not visible,) and incomprehensible (not comprehensible) work their way into the common vernacular of theology. This is not by accident, theologians have long struggled to define exactly who God is, and sometimes you can accomplish more by saying what God is not.
So when we use a word like eternal, it is best understood by what it is not.
Before I can say what God is not, I would try to quickly define what time is. What exactly is time? We rarely think about it unless we are short of it. Time is simply a way of measuring motion between a beginning point and an endpoint. How did someone decide exactly the rhythm your second hand on your watch would have? Firstly, they measured the amount of time it takes for the Earth to complete one orbit around the Sun, but instead of saying one orbit is 31,536,000 little bits of time, we split that up into rotations of the planet. So there are 365 rotations in one orbit, and divide that rotation up into consecutively smaller bits, slap some labels on, like minute and year, and violà. You have our time measurement system as we know it today. The one thing consistent about what all of those measurements have in common is that they are signifying change.
So when we say that God is eternal, we don’t mean that God has experienced infinitely more moments than we have, for, to experience time, one has to experience change. Where there is no change, there is no time, and the Lord our God never changes. He is the one who is known as the I Am. Not the I was, or the I am becoming. He simply is.
So God is not eternal because He simply lives for infinitely longer than we do, He is eternal because He exists outside of temporal constraints altogether. His existing is not coming and going, as ours is, but it is a single standing limitless unmoving instant of existence. He doesn’t have a life through which He is passing, but He is the life by which He is living. God does not fit into a category of things which are eternal, He is eternality itself.