I was having an interesting conversation with an atheist over at Wasteland Of Wonders [an Atheist/Agnostic Website and Message board] when the topic of ultimate purpose came up!
Now most believers in God, myself included, seem to say that if the universe just is, then it becomes a big pointless absurdity, almost like a sick joke!
However if God exists then the universe and everything in it has a purpose, but the fellow over at Wastelands of Wonder with whom I was chatting said the following :
"Okay then, what is the ultimate purpose of God's existence? Don't you just have the same problem with theism, but pushed back a level? If God "just is," what purpose then is there for your existence?"
I have to say this question reminds me very much of the infinite regression problem of "If God exists then who made God?!"
The best I could think of was that God contains an explanation for himself and that was it! This question truly had me bedazzled and I was scratching my head looking for a decent, non-cop-out explanation! [Like God explains himself]
So may I ask, if you were asked this question by someone what would you say?
Thank you for the interesting question! Let me try to answer it this way:
First, there would seem to be an important difference between the two questions, "Why does the universe exist?", and "Why does God exist?" Today, most scientists and philosophers believe that the universe had a beginning; it is not eternal. However, if God exists at all, He exists necessarily and is therefore eternal. Thus, even though each question is asking WHY something exists, they are each asking this about very different kinds of things.
Second, it's important for us to remember that purposes can only exist within a mind. The dictionary on my desk defines "purpose" as follows: 1. something one has in mind to get or do; plan; aim; intention. 2. object or end for which a thing is made, done, used, etc. Clearly, nothing which lacks a mind can have purposes of this sort. Whatever purpose there is for the existence of impersonal things must come from intelligent, purposeful beings. As a general rule, such beings would also be personal. Here I am thinking primarily of man, but also of God and the angels if they exist. Of course, some higher animals may have what might be described as very limited sorts of purposes for some of the things which they do. But generally speaking, purposes are the products of intelligent, personal beings.
Thus, if the universe is simply a "brute fact," and was not brought into existence by a purposeful, intelligent being, there can be no ultimate purpose for its existence. If nothing exists outside the universe then clearly, going back to the previously given definition of purpose, there can be no object or end for which the universe came into existence. The universe can only have some ultimate purpose if it was created by an intelligent being who, in fact, had some purpose in making it.
However, when we come to the question which you were asked, "What is the ultimate purpose of God's existence?", we need to pause and consider exactly what we are being asked. I think you are correct in seeing this question as a variant of that other, often-asked question, "Who made God?" While such questions can be asked, I honestly doubt whether they are truly meaningful.
In the case of the question, "Who made God?", the questioner seems to be assuming that whatever exists requires a cause of its existence. But this is not true. Actually, it is only what BEGINS to exist that requires a cause. The universe began to exist; therefore, the universe requires a cause of its existence. But God never began to exist; He is eternal. It is therefore meaningless to ask "Who made God?", for what is really being asked is something like "Who made the Unmade Maker?", or "Who created the Uncreated Creator?" Clearly such questions are meaningless.
I believe that the question, "What is the ultimate purpose of God's existence?" or "Why does God exist?", is probably a similar sort of question. If the Christian God exists, then He is eternal. It is therefore unnecessary to posit a cause of His existence. Furthermore, if the Christian God exists, then He is the Creator of everything (other than Himself, of course!).
But now go back to our definition of "purpose" mentioned earlier and remember that, since God is the eternal, Uncreated Creator of all that exists, there was clearly no one other than God who might have had a purpose for bringing Him into existence. Additionally, it would also be meaningless to ask what purpose God had for bringing Himself into existence. The notion of a self-caused being is absurd. In order for a being to cause its own existence, it would first have to exist - which is obviously absurd. But if the purpose of God's existence cannot be explained by reference to an intelligent, purposeful being other than God, and if it can also not be explained by referring to a self-creative act of God Himself, I conclude that the question is probably meaningless.
Thus, while one can meaningfully ask about God's purpose(s) in creating the universe (and thus about the reason WHY the universe exists), one cannot meaningfully ask this question about God Himself. Probably, the question is simply meaningless. But if not, we could not possibly know "the ultimate purpose of God's existence" unless He tells us--and so far as I'm aware, He hasn't done so.
Hope this helps.