Immortalist wrote:

To say that god caused the universe to exist is to argue that man can
never comprehend the existence of the universe. The theist demands a
causal explanation of the universe and then fails to provide an
explanation. Even if a supernatural being did exist, the "problem" of
existence would be as puzzling as before. After all, how did it create
existence from nonexistence? "Somehow" is not an explanation, and
"through some incomprehensible means" is a poorer explanation still.
The theist is trapped in a dilemma of his own making-the "mystery" of
existence-and he must confront an unintelligible universe.

Are the premises of the first-cause argument true? Does the universe
require a causal explanation?

In considering the causal argument as a whole, one contradiction
immediately stands out. The first premise of this argument states that
everything must have a cause, and the conclusion asserts the existence
of an uncaused supernatural being. But if everything must have a cause,
how did god become exempt? Professor John Hospers points out the
contradictory nature of the first-cause argument:

. . . the causal argument is not merely invalid but self-contradictory:
the conclusion, which says that something (God) does not have a cause,
contradicts the premise, which says that everything does have a cause.
If that premise is true, the conclusion cannot be true; and if the
conclusion is true, the premise cannot be. Many people do not at once
see this because they use the argument to get to God, and then, having
arrived where they want to go, they forget all about the argument ...
if the conclusion contradicts its own premise, we have the most damning
indictment of an argument that we could possibly have: that it is
self-contradictory.2

In The Necessity of Atheism, David Brooks makes a similar criticism:

By predicating a First Cause ...the theist removes the mystery a stage
further back. This First Cause they assume to be a cause that was not
caused and this First Cause is God. Such a belief is a logical
absurdity, and is an example of the ancient custom of creating a
mystery to explain a mystery. If everything must have a cause, then the
First Cause must be caused and therefore: Who made God? To say-that
this First Cause always existed is to deny the basic assumption of this
"Theory." Moreover, if it is reasonable to assume a First Cause as
having always existed, why is it unreasonable to assume that the
materials of the universe always existed? To explain the unknown by the
known is a logical procedure; to explain the known by the unknown is a
form of theological lunacy.

The contradiction in the causal argument stems from its basic flaw: its
demand for a causal explanation of the universe, the totality of
existence.

When one asks for the cause of something, whether it be an entity or
event, one is asking for the entity or action of an entity (prior
event) that caused it. Causal explanation is possible only within the
context of existence. Nathaniel Branden writes:

Within the universe, the emergence of new entities can be explained in
terms of the actions of entities that already exist. ... All actions
presuppose the existence of entities-and all emergences of new
entities presuppose the existence of entities that caused their
emergence. All causality presupposes the existence of something that
acts as a cause. To demand a cause for all of existence is to demand a
contradiction: if the cause exists, it is part of existence; if it does
not exist, it cannot be a cause. . . . Causality presupposes existence,
existence does not presuppose causality. . . . Existence-not
"God"-is the First Cause.

This passage demonstrates that the causal argument drops the
epistemological context that gives meaning to the concept of causality.
"What caused the universe?" is an absurd question, because before
something can act as a cause, it must first exist-i.e., it must first
be part of the universe. The universe sets the foundation for causal
explanation and cannot itself require a causal explanation.

The primacy of existence is illustrated in science by the principle
that matter can neither be created nor destroyed. J. S. Mill hinted at
the above objection to the causal argument when he stated that, "As a
fact of experience: ...causation cannot legitimately be extended to the
material universe itself, but only to its changeable phenomena. . . ."
Chapman Cohen, a noted English atheist, commented that discussing
causality outside the context of the universe "is like discussing a
bird's flight in the absence of an atmosphere."

The universe, then, has always existed and always will exist. Some
theists find this difficult to accept, and they argue that god makes
the universe easier to understand. Yet, while the theist complains of
difficulty accepting the notion of an eternally existing universe,
consider his alternative. We must conceive of a supernatural,
unknowable, eternally existing being, and, moreover, we must conceive
of this being creating matter from the void of nonexistence. It is
strange that those who object to the idea of eternal matter display
little difficulty in accepting the creation of something out of
nothing. While the idea of an eternal universe may be initially
difficult for some people to assimilate, the theist's alternative is an
exercise in fantasy.

Until the theist is able to discuss causality in terms of nonexistence,
his demand for a cause of the universe will remain nonsensical-and he
will be unable to escape the contradictory nature of the causal
argument.

Atheism: The Case Against God
George H. Smith
http://www.amazon.com/gp/product/087975124X/

You have simply renamed God, "Existence." As stated explicitly, above:
"Existence-not "God"-is the First Cause.

First Cause == Necessary Existent, and that, we call "God." It doesn't
matter too much what word you use for such a thing. But your main
objection appears to be to the use of a particular word to name that
concept, not to the concept itself.