Many atheists assert that an eternal universe is explanatorily equivalent to an eternal God.  For example, Sagan once asked, “If we say that God has always been, why not save a step and conclude that the universe has always been?”[1]  And just recently, two prominent atheists made the same claim.  In his new book, A Universe from Nothing, Lawrence Krauss writes, “[T]he declaration of a First Cause still leaves open the question, ‘Who created the Creator?’ After all, what is the difference between arguing in favor of an eternally existing creator versus an eternally existing universe without one?”[2]  Victor Stenger agrees with Krauss:

Krauss also describes how cosmology now strongly suggests that a “multiverse” exists in which our universe is just one member.  So, the real issue is not where our particular universe came from but where the multiverse came from. This question has an easy answer: the multiverse is eternal.  So, since it always was, it didn’t have to come from anything.

And, to bring religion into the picture, one could ask: Why is there God rather than nothing?  Once theologians assert that there is a God (as opposed to nothing), they can’t turn around and ask a cosmologist why there is a universe (as opposed to nothing). They claim God is a necessary entity. But then, why can’t a godless multiverse be a necessary entity?[3]

Apart from the fact that all of the evidence points to a temporally finite universe and a temporally finite multiverse (if a multiverse even exists), is it true that an eternal universe is explanatorily equivalent to an eternal God?  No, not at all!  An eternal universe would be a brute fact that lacks an explanation for its own contingent existence, whereas God—as a metaphysically necessary being—provides an explanation for both His own existence and that of the contingent world.  Edward Feser, in response to Krauss, explains why God is not explanatorily equivalent to an eternal universe:

In general, classical philosophical theology argues for the existence of a first cause of the world—a cause that does not merely happen not to have a cause of its own but that (unlike everything else that exists) in principle does not require one. Nothing else can provide an ultimate explanation of the world. …
“[T]hings in the world can change only if there is something that changes or actualizes everything else without the need (or indeed even the possibility) of its being actualized itself, precisely because it is already ‘pure actuality.’ Change requires an unchangeable changer or unmovable mover.”
“[E]verything made up of parts can be explained only by reference to something that combines the parts. Accordingly, the ultimate explanation of things must be utterly simple and therefore without the need or even the possibility of being assembled into being by something else. … For Leibniz, the existence of anything that is in any way contingent can be explained only by its origin in an absolutely necessary being.
But Krauss simply can’t see the ‘difference between arguing in favor of an eternally existing creator versus an eternally existing universe without one.’ The difference…is that the universe changes while the unmoved mover does not, or, as the Neoplatonist can tell you, that the universe is made up of parts while its source is absolutely one; or, as Leibniz could tell you, that the universe is contingent and God absolutely necessary. There is thus a principled reason for regarding God rather than the universe as the terminus of explanation. 
One can sensibly argue that the existence of such a God has not been established. (I think it has been, but that’s a topic for another day.) One cannot sensibly dispute that the unchanging, simple, and necessary God of classical theism, if he exists, would differ from our changing, composite, contingent universe in requiring no cause of his own.[4]

As Feser noted, there are principled arguments for positing a personal creator God, and such a being has great explanatory value.  In contrast, there are no good reasons for positing an eternal universe, and even if the universe was eternal, it lacks explanatory value.  That, Mr. Krauss, is the difference between positing an eternal universe and an eternal God, and why, Mr. Sagan, we cannot just save ourselves a step and consider the universe to be eternal. 

[1]Carl Sagan, Cosmos, 257.
[2]Lawrence Krauss, A Universe from Nothing: Why There is Something Rather than Nothing (New York: Simon & Schuster Inc., 2012), xii.
[3]Victor Stenger, “Nuthin’ to Explain”; available from; Internet; accessed 20 April 2012.
[4]Edward Feser, “Not Understanding Nothing”; available from; Internet; accessed 16 May 2012.