God specifically does not have a beginning, or an ending. However the human mind is not good at grasping concepts like eternity, infinity, or similar absolutes.
We can't
experience something unlimited; e.g. there isn't enough time in the day to experience an event of infinite duration.
However, we don't have any real issues understanding concepts like eternity or infinity. E.g. Set theory, calculus and other branches of mathematics have no problems dealing with infinity; Cantor developed transfinite numbers; ZFC includes the axiom of infinity, and so forth.
Further, one rejoinder to the cosmological arguments is that the energy that makes up the universe
can be "eternal," i.e. neither created nor destroyed. It wasn't always in its current form, but it always existed.
Or, we can say that one reason that creation
ex nihilo is impossible is because "nothingness" itself is a contradiction, and makes no sense. E.g. "nothingness" cannot be the "absences of something," because then "nothingness" has a property; and Nothing cannot actually have any properties (including the property of "unable to hold any properties"). The term "nothingness" is actually a reference failure, because there is no thing to refer to. "Nothingness" cannot exist, because if it exists then it is classified as "Something," but "Nothingness" is the total absence of all properties including thing-ness / existence. I.e. if Nothing exists, then it cannot be nothing, because it exists.
Even a vacuum is not Nothingness, because it has properties (temperature, volume, spatial extents). Though we should note that in contemporary quantum theories, we can never have a
total vacuum, as that would violate the Uncertainty Principle, and would require that the region of space be at absolute zero. There will always be some zero-point energy in a given region of space.
At best, "nothingness" is a linguistic negation, that only makes sense in a metaphorical sense. When I say "there is nothing in the box," I do not mean "there is a total absence of existence inside the confines of the box." I don't even mean "the box is a vacuum." I mean "there are no ordinary everyday objects in the box."
I.e. perhaps there is Something, because it is impossible for there to be Nothing. In which case, the universe logically
must have always existed in some form; and creation
ex nihilo is impossible because
nihilo is not possible. In which case, there's no logical requirement for a supernatural entity to bring all the universe's energy into being.
Note that this does not disprove the existence of a supernatural creator-deity, only that the cosmological arguments are not valid, as "the energy that makes up the universe has always existed" may not just be logically sound, it's logically necessary.