I'm going to start off with a few definitions. These are my own definitions, and they may or may not be reflected anywhere else or by anyone else.
atheism: the absence of religion or the absence of belief in a deity or deities (literally a-theism: without religion). atheism is divided into two types of atheist: Agnostic atheists, and the other kind of atheist, who I am going to call an affirmative (non-Agnostic) atheist.
agnosticism: the refusal to commit to a belief in a deity or the abscence of a deity. In essence, the ultimate in open-mindedness.
affirmative atheism: atheists who are not agnostic are affirmative atheists. They believe that there is no deity or higher power. This is itself a religious belief, and like any religious belief, requires a certain amount of faith in the unknown.
Atheism (capital A): an organized sect (similar to a religion) that professes the nonexistence of deities. The difference between atheism and Atheism is organization. Atheists are organized and communal; atheists are not.
All of this can be rather confusing and is often confused, by religious people and atheists alike.
Overall, I think the distinction between agnosticism and atheism is less important than the distinction between atheism and Atheism. In certain elements of the news media and the political class, atheists are viewed as Atheists. Similar to the Cold War era fallacy that all communists were united (and joined with socialists) in a single massive organization, this is false. Most atheists are simply individuals who for whatever reason don't believe in a god or gods. They may have been raised without religion, or they may have come to their beliefs independently. A(a)theists are less likely to assemble than religious people, and they are less likely to form a community around their lack of belief because their lack of belief is not central to their identity or how they live their lives. Unlike a Christianity, Islam, or Hinduism, atheism requires nothing of people who practice it because one does not practice it. It simply is. (Of course, there are many Atheists, and they do support some kind of Atheist community and dogma.)
This brings me to science. Science and religion are often pitted as adversaries. People say that science wishes to destroy religion, or that science is intrinsically Atheist (with a capital A). First of all, science is not atheist, nor is it Atheist. It is necessarily agnostic in all matters including theology. Let me explain that. The nature of science is to be unsure--to challenge what others believe and what you believe. This idea that all beliefs must be based in evidence is the core of science and is also the motivation of agnosticism. Conversely, religion and affirmative atheism are both founded around the premise of faith in the unprovable or unknowable. This does not mean that all scientists are atheist or even agnostic in their personal lives. I happen to know several engineering and science students who are devout Christians. What it does mean is that an educated person, particularly a scientist, does not hold his or her religious beliefs above question, and is not averse to putting these beliefs in the context of what they and others observe of the world. It is hypocritical for one to accept some scientific knowledge (chemistry, electricity, cosmology, etc) but not others (evolution, geology, etc) without providing evidence against one claim (e.g. evolution) in favor of another (e.g. creation). To do so would be hypocritical and illogical.
This means that all educated people must answer hard questions about their beliefs. Some people are able to reconcile their knowledge of the world around them (science) with their beliefs about what they do not know (religion). As I mentioned earlier, I know many intelligent religious people who focus their religious beliefs on the things they do not know. They don't let their beliefs interfere with what science has answered. Others cannot reconcile their religion with science. Some become affirmative atheists. I must stress here that the premise that there is no god has no more evidence behind it than the premise that there is one, so affirmative atheism is itself a belief. Some become agnostic, refusing to commit to either premise. There is a fourth group, and they are a thorn in the side of those who view science as progress. These people do not question their beliefs. Rather than construct a system of beliefs that can live in harmony with the human body of knowledge, they deny that knowledge. These are the people who refute evolution. These are the young-Earth creationists. These are the people who challenge global warming with arguments from Genesis. They are suspicious of science, and they demonize it. To them, science is a conspiratorial effort to systematically dismantle religion. Were this true, they would have every right to vilify science and scientists. Fortunately for us all, it is not.
Science is not the adversary of religion but the adversary of the unknowable. Science attempts to explain and answer the questions that arise from everything around us. Science explained lightning and sunrises, which were once purely the domain of religion. Now, science is explaining where life came from, which is still in the domain of religion for many. Scientists (or at least most of them) do not set out to disprove religion. Religion sought answers to things that were unknowable. Some of these things have since become known, and what we come to know sometimes conflicts with religion. That means religion was wrong about some things, certainly not for the first time and probably not for the last. Of course, it is fallacious to suggest that disproving part of religion disproves it all. Science can never explain everything. The realm of the unknowable is shrinking, but slowly. There is still much for religion to explain, but in committing to religion, understand that you are putting your faith in something unknown. Be prepared for the day that we can know it, and be prepared for the possibility that you were wrong. Accept this graciously, and reexamine what you believe.
