*Read previous post before reading this one.*
There is a difference between the question, Is existence necessary?, and Does X exist necessarily? The first question asks about existence in general, whereas the second asks about the existence of some specific thing within the larger domain of existence.
Regarding the first question, is it necessary that something exist? The answer to this question depends on whether one is speaking of historical possibility, or metaphysical possibility. Historically speaking, the answer is an emphatic yes. Something must exist, and must exist eternally. Why? Because something does exist. If there was ever a time when absolutely nothing existed, absolutely nothing would “exist” now, because nothing has no potentiality to ever become something. And yet there is something, so we know there has never been a time when nothing existed.
But from a metaphysical perspective, there is no reason to think existence itself is necessary. We can conceive of absolute nothingness. Furthermore, there is nothing logically incoherent about the concept of non-existence. Existence, then, is not necessary, but contingent, and contingent things require an explanation for their existence. What, then, is the explanation of existence? Why is there something, rather than nothing?
The second question is quite different. It does not ask whether existence itself is necessary, but whether the existence of some particular X is necessary. In cosmological arguments, X stands for the universe. Does the universe exist necessarily?
Some atheists assume the answer to this question is wrapped up in the first: Since something must exist eternally, the universe must be eternal. While it is true that something must have always existed, why think the universe is that something? Not only are there compelling scientific and philosophical reasons to think the universe exists contingently, but this begs the question in favor of atheism. It assumes materialism from the start (i.e. the universe is all that exists), reasoning that since something must be eternal, and the universe exhausts reality, then the universe must be eternal. But that the universe exhausts reality is what stands to be proven.
Secondly, thinking the universe exists by a necessity of its own nature is a grandiose claim that few philosophers are willing to countenance. To say the universe exists by a necessity of its own nature does not merely affirm the necessity of a universe in general, but the necessity of our universe in particular. It is an affirmation that the very fundamental particles of our universe–quarks, neutrons, electrons, etc.–are necessary, not just in kind, but in number and arrangement as well. But this is absurd. There is no reason to think the universe could not have been composed of a different set/number of fundamental particles, arranged in a different way, operating by a different set of physical laws, resulting in a totally different kind of universe. In fact, it is quite possible to conceive of a physically empty universe, or no universe at all. There is no physical or logical law that requires the universe to exist. So modal logic alone demonstrates the universe is not necessary. It is contingent, meaning it is metaphysically possible that it might have never been.
We can agree with the atheist that existence is necessary as a historical fact, and that the universe does not exist necessarily. But these two truths, coupled with the scientific and philosophic evidence for the finitude and contingency of the universe, provide a strong argument for a personal God. Something must exist eternally, and since the universe is not that something, it must be something else. Whatever caused the spatio-temporal-material universe to exist must itself be eternal, non-spatial, and immaterial.
Only two things fit such a description: abstract objects, or an unembodied mind. Since abstract objects are causally impotent by definition, they cannot be the cause of the universe, and thus are unlikely to be that which has always existed. That leaves us with an unembodied mind as the eternal reality. This makes sense. Not only are we are intimately acquainted with the idea of minds creating things, but it also makes sense of the design and order we see in the universe. An intelligent agent best explains why the universe exists as it does. Since an eternal, non-spatial, immaterial, intelligent mind is what most mean by “God,” it is best to conclude that God is that which exists eternally, and hence necessarily. He is a necessary being, who contains within Himself the sufficient reason for His own existence, and is the cause of everything else.