If God is self-existent, He is eternal. That is, He utterly transcends in essence, life, and action the relations of time. If He were a temporal being He would have a beginning, even though His origin coincided with the origin of time. Such a being cannot be uncaused or self-existent. Moreover, if He were temporal, whether in essence or in action, He would be conditioned externally and limited by temporal relations and sequences, and His self-realization would at each moment be partial and imperfect.
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Real eternity is infinite and non-temporal. It is not, in particular, an indefinite expansion of temporal duration, but is that attribute of God which, although timeless, is conceived when we contemplate the infinity of God from the point of view of the temporal.
The Being and Attributes of God, Francis J. Hall, D.D., New York:Longmans, 1918, p. 253-254.