A short explanation of Thomistic metaphysics, and how it relates to God.

   When something is said to be, we tend to mean this in two ways, which 
should each be distinguished.  In one sense, as when we say that "The keebler
elf is short", we are talking about how a thing is understood, it's whatness,
or, as Aquinas says, it's "essence".  So, in one sense of "to be", we refer to
what makes a thing what it is, and what is it's essence.

  In another sense, as when we say that "The car is." or "The car exists."
(synonymous meanings), we refer to the existence of the thing simply.  The
meaning of "car" is assumed understood, and the statement asserts that
whatever this essence is, it exists as a real thing.  This sense of being is
called "being" simply, or "existence".  We confirm each of these senses of
being through perception.  The effect of an existant thing on our senses tells
us _what_ a thing is (it's essence), while the fact that it affects our senses
at all tells us _that_ a thing is (it's existence).  For this reason, we can
also call essence "whatness" and existence "thatness".

  Now we also find that essence and existence are _distinct_ properties.  We
confirm this by noting that while we can define the essence of this computer
I am typing at (we can say that it is white, and having keys, and buttons,
and markings, and made of atoms, and powered by electricity, etc,etc,etc) and
we can also define Ernie the Keebler Elf (he is short, and wears green, and
makes crackers, and is friendly, and a pleasant smile), but we find that
only one of the above has existence (I'll leave it as a problem to the reader
as to which it is ;)).  Likewise, we can define two refrigerators, identical
except that one is in apartment 205 and the other in apartment 705, and yet
(as I happen to know) only one of them actually exists.  And yet another
way I've heard it put is that while the refrigerator we perceive may have
it's essence brought into the mind, it's existence remains distinct (the
frig is not the same as it's essence).

   Moreover, we find that existence is not only distinct from, but is _prior_ 
to essence.  This we confirm as we did above, noting that to be, an essence
(like refrigeratorness, or Ernieness) requires existence to be, while essence
without existence is merely nothing (though perhaps is an idea).  Note that
unlike Platonic thought, no existence is attributed to essence necessarily
(they are distinct) and moreover that, unlike Islamic thought, essence is
not considered prior to existence (as you can't get something from nothing).
Lastly, note that while essence requires existence to be, existence does not
require existence to be (it is it's own existence); that is to say that
while essence contingently exists, existence simply exists.

  The last topic is that of the nature of existence itself.  Essence defines
the "whatness" of a thing, and is understandable as it is. Existence however,
being distinct from essence, seems to act as a realizor (thanks to the 
objectivist folk for this great term) of essence.  We find then that existence
is without regard for the essence that it realizes; by this I mean that 
since existence is prior to corporeality, it is therefore not necessarily
corporeal, nor necessarily incorporeal-- it is _without_regard_ for 
corporeality.  Existence merely realizes the essence of corporeality in things,
and is not the same as that corporeality (as shown above).  Moreoever, we find
that existence is without regard for temporeality (existence realizes this
thing _now_, and thus, since existence must be prior to time, it must also
be without regard for time, thus being in all times).  

  Moreover, we find that (in general) essence limits how a thing may be.
Borrowing from the tenant of reason (that nothing cannot both BE and not BE),
we find that whenever a thing is corporeal, we limit it by saying that it is
NOT incorporeal at the same time.  Ditto for temporeality, color, shape, 
and all other essences which we may perceive or understand.  Existence is thus
(from this, and the above) without regard for any of them, but may realize any 
of them.  Further, since to be irrational (like a rock) is an essence, 
existence must be without regard for rationality (being capable of, and 
realizing both).  

  Note that all of the things we learn about existence we learn through 
negative knowledge.  We can know what existence is NOT, but not completely 
what it IS (since existence may realize all essences, and we are only aware of 
those essences we perceive or may imagine or dream up).  For this reason, 
Aquinas calls existence (or being) transcendential.

  So, then, you may guess at this point that the pure undetermined, existence
prior to essence (existence whose only essence IS it's existence) is, in
fact, the omnipotent (unlimited in what it may be), omniscient (unlimited in
what it may know), all-giving (being constantly granting existence),
eternal (being prior to time) God. It should be clear now why we say that 
God is not male or female (He is without regard for it and so forth), why we 
say that God is not a man (as this would be to limit existence with an 
essence other than itself), and why we say that God is not two or three 
(since to count things, there must be a distinction, and a distinction is the
same as an essence, which is not bound to existence simply).