Historically, God's eternity has been seen as an aspect, or corollary, of his infinity. Since God's essence is not limited by anything external to himself, it follows that his essence is not limited by time. This is a basic truth in theology; God is essentially eternal
Recent discussions about God's eternity, however, seem to confuse and equivocate with respect to just what eternity is. In some discussions, eternity seems to be equated with a context or environment in which Gos operates as God. Eternity is seen primarily in terms of God's relationship to time.
Though this sheds light on a biblical understanding of eternity, it is not the way a discussion of eternity should begin. The way to think about eternity is to articulate what it is first in relation to God and then in relation to time. First, eternity is an essential property of God quite apart from time. Eternity is God's unlimited essential duration. It is the affirmation that God, unlike time, has always been; he never began to exist, nor was his existence (eternally) caused, nor will it end, but it simply is, without past, present, or future (just as infinity simply is without spatial categories). Eternity, therefore, references something of the duration of god's "is-ness". It points us to the fact that God's necessary existence has always been, and has always been without temporal categories. (212-13)
I suppose such a discussion may be over-the-top and unhelpful for some. But this type of mind-stretching thinking is helpful to me. It reminds me of God's transcendence; we can barely find words to describe him let alone understand him. It also causes awe. God's greatness has many facets and investigating them all is a privilege of great benefit to us. I'm thankful for string minds like Oliphint's who can wrestle with these things and explain them to the rest of us.