... as summarized by the great apologist Frank Sheed:
If we consider the universe, we find that everything in it bears this mark, that it does exist but might very well not have existed. We ourselves exist, but we would not have existed if a man and a woman had not met and mated. The same mark can be found upon everything. A particular valley exists because a stream of water took that way down, perhaps
because the ice melted up there. If the melting ice had not been there, there would have been no valley. And so with all the things of our experience. They exist, but they would not have existed if some other thing had not been what it was or done what it did.
None of these things, therefore, is the explanation of its own existence or the source of its own existence. In other words, their existence is contingent upon something else. Each things possesses existence, and can pass on existence; but it did not originate its existence. It is essentially a receiver of existence. Now it is impossible to conceive of a universe consisting exclusively of contingent beings, that is, of beings which are only receivers of existence and not originators. The reader who is taking his role as explorer seriously might very well stop reading at this point and let his mind make for itself the effort to conceive a condition in which nothing should exist save receivers of existence.
Anyone who has taken this suggestion seriously and pondered the matter for himself before reading on, will have seen that the thing is a contradiction in terms and therefore an impossibility. If nothing exists save beings that receive their existence, how does anything exist at all? Where do they receive their existence from? In such a system made up exclusively of receivers, one being may have got it from another, and that from still another, but how did existence get into the system at all? Even if you tell yourself that this system contains an infinite number of receivers of existence, you still have not accounted for existence. Even an infinite number of beings, if no one of these is the source of its own existence, will not account for existence.
Thus we are driven to see that the beings of our experience, the contingent beings, could not exist at all unless there is also a being which differs from them by possessing existence in its own right. It does not have to receive existence; it simply has existence. It is not contingent: it simply is. This is the Being that we call God.
All this may seem very simple and matter of course, but in reality we have arrived at a truth of inexhaustible profundity and of inexhaustible fertility in giving birth to other truths.