“The existence of God can be proved in five ways. The first and most obvious proof is the argument from change (ex parte motus). It is clearly the case that some things in this world are in the process of changing. Now everything that is in the process of being changed is changed by something else, since nothing is changed unless it is potentially that towards which it being changed, whereas that which changes is actual. To change something is nothing else than to bring it from potentiality to actuality, and a thing can be brought from potentiality to actuality only by something which is actual. Thus a fire, which is actually hot, makes wood, which is potentially hot, to be actually hot, thus changing and altering it. Now it is impossible for the same thing to be both actual and potential in the same respect, although it may be so in different respects. What is actually hot cannot at the same time be potentially hot, although it is potentially cold. It is therefore impossible that, in the same manner and in the same way, anything should be both the one which effects change and the one that is changed, so that it should change itself. Whatever is changed must therefore be changed by something else. If, then, whatever is changing it is itself changed, this also must be changed by something else, and this in turn by something else again. But this cannot go on forever, since there would then be no first cause to this process of change, and consequently no other agent of change, because secondary things which change cannot change unless they are changed by the first cause, in the same way as a stick cannot move unless it is moved by the hand. We are therefore bound to arrive at a first cause of change which is not changed by anything and everyone understands that this is God.”
The above is a cosmological argument for the existence of God, from roughly a millenium ago, and one that still holds some weight today, even if its specifics are argued. This is based on the principle of Cause & Effect (which is both a scientific concept and one that has been espoused by various schools of philosophy for millenia), and the argument goes somewhat like the following, with my own (half-formed) musings added in for good measure:
Something is caused (and must be caused) by something else. We know this from observation alone, as an effect cannot exist without a prior cause for that effect, such as a ripple in a pond requiring some cause, perhaps a stone being dropped within it, or an earthquake being caused by the plates beneath the earth shifting, and the earthquake is in itself the cause of buildings collapsing, and the buildings collapsing are in turn the cause of people being injured, etc. So, the cause of one effect is the effect of a prior cause, and so on, ad infinatum. However, Aquinas postulates that there must be an initial cause of all these subsequent causes, and this he terms "God". I agree with him to some extent here, in that I feel there is indeed one First Principle from which all others are derived, even if that First Principle is indescribable, infinite, and eternal. Some argue against this by asking why there cannot be a series of infinite causes, and to this I say: there is - and this series of infinite causes is God, who is infinite and the Cause of All (and also the Effect, by extension [or emanation], but that's a different topic entirely). If there is a series of infinite causes, and we call this "God", then, by logical conclusion, "God" is the First Principle, which is this series of infinite causes while simultaneously being an initial (infinite) cause, thus the reality of God as I always find him: a paradox.
To illustrate this better, let us use the following example:
A = God/Big Bang
B = Cosmos
C = Earth
D = Humanity
D is the Effect of Cause C, which is the Effect of Cause B, which is the Effect of Cause A. Logically, we have a First Principle, even if it is merely postulated due to our lack of understanding or inability to grasp anything beyond that. Since humanity exists, we know that its existence is the effect of some cause, which we can possibly generalise as "Earth", since without this planet none of the life forms here could exist, and humanity could not have evolved from them. The Earth, likewise, must be the effect of something else, which is, let's say, the Cosmos, or the Universe at large, through the various workings of the Solar System and the formation, etc. of stars. This, likewise, was established by some other cause, which Science currently calls "The Big Bang". We can call this "God", for now. If we accept that "The Big Bang" was the cause of our universe, then we have a First Principle. We can effectively label this "God", as he is generally conceived of as being the "Source" of All, or the First Principle (a label like "God" and "Big Bang" remains a label).
If we find out, however, that there was something before the Big Bang, the Big Bang ceases to be the First Principle, becoming a Second Principle to whatever new information we found created that effect (God, perhaps?). Likewise, recognising that there is indeed a Demiurge, who is not the First Principle (even if he thinks he is) means that this conception of "God the Creator" is not the First Principle, but the Second, and the First Principle remains "GOD" as he truly is. God is, in effect, the First Principle in actuality, even if the subsquent principles are rearanged constantly to fit the arising of new information. God is the Source of All, the Primal Cause, the Fullness, the Centre from which everthing Emanates and Radiates, and there cannot be anything beyond or above him, because if there was, then whatever he is ceases to be "GOD" (or "the First Principle") and becomes a secondary principle to that which is above him, which is "GOD" and "the First Principle". So, even if we wrongly label something with these terms (such as the Demiurge), it doesn't defeat the reality that there is one Origin to which all of this goes back to (and all of this goes forward to in the End), and we label this: God.
A = God
B = "God" [i.e. Demiurge]
C = Cosmos
D = Earth
E = Humanity.
These thoughts are not full-formed in my head, so I am still musing at this point, but feel free to comment and add your own thoughts.