@Frank Apisa,
Well, Frank, let me use Richard Dawkins as an example. He is FAR from a good example, but I'll just accept that self-imposed handicap. I don't think there's any question that Dawkins is intelligent. There are serious questions about whether he is actually "sophisticated" in his analysis of, and pronouncements about, his personal theology, but let's leave those aside.
For Dawkins, I think it comes down to something like this:
1. We don't need to posit the existence of a god for anything. We can find reliable answers to all relevant questions in nature, for example, in neo-darwinistic evolutionary theory, as concerns the issue of "how we came to be." (Extreme arrogance on his part, to be sure, but.....).
2. Using Occam's razor as a guiding principle, it is then senseless to posit an unnecessary god.
3. Furthermore, the very existence of extreme evil in the world empirically disproves the existence of the type of god that most people on this planet believe in. This suggests that the "desire" to believe in god, not the facts themselves, is what leads people to believe in that unnecessary hypothesis.
Do I buy this argument? No, not personally. But nor do I believe that it is nothing more than a "blind guess." Dawkins has thought (however imperfectly) about this topic long and hard, for many years. He is not stupid. One should at least listen to what he says, whether they end up accepting it as "true," or not.