"Ever since the creation of the world his [God's] eternal power and divine nature, invisible though they are, have been understood and seen through the things he has made. So they are without excuse..." (Romans 1:20, NRSV)
In other words, the fact that there is a creator is self-evident. The intricate design and beauty of everything we see around us, shows that the world was created.
Many people object that the theory of evolution sufficiently explains our origins and existence. I disagree. While I am not going to attempt to defend a young-earth creation science position, I believe that the theory of evolution has several serious shortcomings (at least in an atheistic framework). Perhaps the most significant is this:
I do not know of a single instance in nature in which order arises from disorder.
(And no, a growing life form is not a valid counterexample, because the order arises from what is programmed in the DNA.)
The closest example I have seen offered is naturally occuring seives or sorting functions. Richard Dawkins offers the example of a seive on pages 43-44 of his popular book The Blind Watchmaker (Norton, 1987). Material is placed on a seive, the seive is shaked, and the material is sorted - smaller material falls though, larger pieces stay on top - and so order arises from disorder. He suggests the "sorting" of pebbles on a beach (smaller pebbles further up the shore) as an example of a naturally occuring seive - order arising from disorder.
But the example is flawed. In both cases, there is order originally present (in the seive), and the amount of sorting (or order in the sorted material) is limited by the amount order in the seive.
Concerning the man-made seive, this is fairly obvious. With a simple seive, only a small amount of order is introduced: the material is sorted into only two groups. A more complex seive will give rise to a more order. An example of a more complex seive is one of those coin sorters they use in banks (or at least they used to). The sorter has several levels of holes: the top one lets through everything except 50 cent pieces, the next level lets through everything else except 20 cent pieces (or 25 cents in America), the next level lets through everything else except 10 cent pieces (or nickels in America), and so on. So although order is introduced to the disordered pile of coins, this order is limited by the complexity of the seive. In other words, order only arises because some other order was previously present.
The same is true for sorting pebbles on a beach. A simple, uniformly sloping beach will give a fairly bland sort - pebbles grading from small to large. A complexly shaped beach can gives a complex sort - for instance, a certain trough may retain only pebbles weighing between 10 and 50 grams. So again, the order introduced into the pebbles is limited by the amount of order originally present (in the shape of the beach).
The interesting thing about this, is that people are often unaware that they use the same argument in reverse. I can invariably find in their writings, phrases like "How could God allow that?" or "What possible reason was there for God creating it that way?" Now, of course, it is they who are invoking personal incredulity.
So how do we get around this? Isn't there anyone capable of rational argument?
The solution, I believe, is to acknowledge that it is OK to use "personal incredulity" arguments. Each of us must decide whether we believe there is a God, and what that God is like. To do that, we must weigh up our knowledge and experience, and come to some sort of a conclusion. In doing that, we will make judgements.
In my opinion, the problems of atheism are far greater than the problems of theism. Or, to take to popular example from the debate over the existence of a creator: I find it easier to believe that there is some as yet undiscovered use for my appendix, than it is to believe that my eye evolved with no divine guidance.