This word has two meanings that should never be forgotten. Most individuals use the word in common speech to refer to time without beginning or everlasting or endless time. In this meaning of the word, God cannot be said to be eternal, for God is not in time and so, is not subject to change or mutability that characterizes all things in time. The second meaning of eternal is to have being outside of time.
Eternal being is beyond our capacity to imagine or conceive in terms that are positive.
Even if we think we can imagine God, we cannot imagine His eternity. The spiritual is simply that which is not material. So, too, the eternal is simply that which is not in time. The eternal is the immutable .