Only the eternal is forever.
[Actually, this is only a necessary truth on some readings of "forever" and "eternal". We need to read "eternal" as compatible with merely futureward eternity, but not pastward eternity. We need to read "forever" as implying infinite futureward temporal extent, so that existing at every time not count as sufficient for existing forever (imagine a world where the timeline has only a finite extent from beginning to end—we don't want to count a being that exists from the beginning to the end of time as existing forever in that world). It's worth noting that the converse of the above Deep Thought might not be a necessary truth. For a timeless being can be eternal, but I am not sure a timeless being counts as existing "forever", except of course in an analogical sense.]