God is commonly portrayed as being eternal; however, there is more than one way to understand the concept of eternal. On the one hand, God may be thought of as everlasting, which means that God has existed through all of time. On the other hand, God may be thought of as timeless, which means that God exists outside of time, unconstrained by the process of cause and effect.
The idea that God should be eternal in the sense of timeless is partially derived from the characteristic of God being omniscient even though we retain free will. If God exists outside of time, then God can observe all events throughout the course of our history as if they were simultaneous. Thus, God knows what our future holds without also affecting our present or our free will.
An analogy of how this might be so was offered by Thomas Aquinas, who wrote that He who goes along the road does not see those who come after him; whereas he who sees the whole road from a height sees at once all those traveling it. A timeless god is, then, thought to observe the entire course of history at once, just as a person might observe the events along the entire course of a road at once.
A more important basis for defining eternal as timeless is the ancient Greek idea that a perfect god must also be an immutable god. Perfection does not allow for change, but change is a necessary consequence of any person who experiences the changing circumstances of the historical process. According to Greek philosophy, especially that found in the Neoplatonism which would play an important role in the development of Christian theology, the most real being was that which existed perfectly and changelessly beyond the troubles and concerns of our world.
Eternal in the sense of everlasting, on the other hand, presumes a God who is part of and acts within history. Such a god exists through the course of time like other persons and things; however, unlike other persons and things, such a god has no beginning and no end. Arguably, an everlasting god cannot know the details of our future actions and choices without impinging upon our free will. Despite that difficulty, however, the concept of everlasting has tended to be more popular among average believers and even many philosophers because it is easier to comprehend and because it more compatible with the religious experiences and traditions of most people.
There are several arguments used to make a case for the idea that God is very definitely in time. God, for example, is thought to be alive but lives are a series of events and events must occur in some temporal framework. Furthermore, God acts and causes things to happen but actions are events and causation is linked to events, which are (as already noted) rooted in time.
The attribute of eternal is one of those where the conflict between the Greek and Jewish heritage of philosophical theism is most obvious. Both Jewish and Christian scriptures point to a God who is everlasting, acting in human history, and very much capable of change. Christian and Neoplatonic theology, however, is often committed to a God who is so perfect and so far beyond the type of existence we understand that it is no longer recognizable.
This is perhaps one indicator of an important flaw in the assumptions which lie behind the classical ideas about what constitutes perfection. Why must perfection be something which is beyond our ability to recognize and understand? Why is it argued that just about everything which makes us human and makes our lives worth living something which detracts from perfection?
These and other questions pose serious problems for the stability of the argument that God must be timeless. An everlasting God, however, is a different story. Such a God is more comprehensible; however, the trait of everlasting does tend to conflict with other Neoplatonic traits like perfection and immutable. Either way, assuming that God is eternal is not without problems.