Often, in discussions of atheism on the internet, an issue arises about the distinction between atheism and agnosticism. Sometimes it takes the form of an atheist pressing someone who prefers to call herself "an agnostic" to accept the label of "atheist". Personally, I think that people should label themselves how they wish, or avoid labels if they want (and can). Still, this issue arises very commonly: I won't even start to provide links to prove it; you'll have to take my word for it, if you haven't had the same experience. So, maybe it's worth trying to get the distinction clear.
First, it seems that there is an overlap between atheism and agnosticism. As I understand it, an agnostic is someone who claims to lack knowledge (as to the existence of God - though the expression is sometimes extended by analogy as in "I'm agnostic about whether Obama made a good choice for Secretary of State").
Theists can't also be agnostic, or if I'm wrong and there is some overlap between theism and agnosticism it certainly cannot be typical. Many theists do claim to have knowledge, even if they also claim that knowledge can be based on something like a leap of faith or the hearing of an inner voice, or the possession of a supposedly innate conviction. They may be wrong about all this - I believe that they are - but they do claim to have knowledge of the existence of their god of choice. Putting it another way, they believe that their belief in the existence of this being is appropriately justified.
As for atheists, I work with the concept that an atheist is someone who has no belief that God or gods exist. Such a person may also say that they do not have knowledge that these beings don't exist. They believe there's no justification for a confident claim either way. There's no inconsistency between being an atheist and also being an agnostic in those senses.
Still, some atheists make stronger claims. To do so, it is not necessary to claim anything as strong as certainty. You can have an appropriately justified belief that falls well short of certainty. It seems to me that you can have an appropriately justified belief that at least some gods don't exist, and at the same time you can have the belief that your belief is justified, so you express it with confidence. E.g., I have what I consider an appropriately justified belief that Zeus does not exist - there is plenty of evidence that Zeus is man-made, essentially a fictional character. For example, the existence of this being is not required to explain lightning and rain, no such being has ever been reliably encountered on Mount Olympus or anywhere else, the stories relating to Zeus are implausible beyond their cultural setting (why is the lifestyle of this being such an ancient Greek one?), and so on. Attempts to save the existence of Zeus from such criticisms would appear ad hoc and even desperate, despite the fact that it is logically possible that a being something like this exists somewhere in the Universe.
In short, I confidently deny Zeus's existence. I think I have appropriate justification for denying that any such being exists. The same applies to all the gods of pagan mythology.
I also confidently deny the existence of anything remotely like the God described in the Old Testament, and on pretty similar grounds to my denial of the existence of Zeus (though the grounds won't always be the same - e.g., on some conceptions of the Old Testament God, he is supposed to have caused events that we can confidently say never happened, such as Noah's flood).
Likewise, though for more philosophical reasons, I confidently deny the existence of the God(s) described in various popular accounts of Christianity. I also deny the existence of the more abstract God of the orthodox Abrahamic tradition of philosophical theology, insofar as this being is supposed to be all-powerful, all-knowing, and also loving and providential. Here, my justification may not be as strong, but the evidence is so strongly against the existence of this being that confidence of its non-existence strikes me as justified. Attempts to get around problems such as the traditional Problem of Evil, the hiddenness of this god, and so on, strike me as ad hoc and desperate. When I throw up my hands and say Unbelievable!, I think this is a rational response.
I'm less sure what to say about the gods of deism and pantheism. I don't believe they exist, so I am an atheist about them in at least that weak sense. It also strikes me that the grounds for claiming they exist are rather weak, and there's an appearance of wishful thinking, or of heuristic bias in favour of seeking ultimate explanations that are couched in terms of agency. Still, I would feel less confident about denying the existence of such beings. When we reach the far ends of explanation for various phenomena, we may think that positing, say, a deist God is just ad hoc, an unnecessary science stopper. On the other hand, we just don't know enough about the ultimate answers to philosophical questions to rule out the existence of the deist God (or, I suppose, the pantheist one).
On balance, I am actually prepared to deny the existence of the deist and pantheist gods, but with a lower level of confidence because I have a lower level of justification for concluding that these beings don't existence. They probably don't, but I could not put a level of probability on it.
None of this worries me too much, because even if I'm wrong and it turns out that such a being exists it can't make any difference to how I live my life. The deist and pantheist gods do not have opinions about, for example, who I've slept with, whether it was a good idea to eat pork and drink wine, and so on. Nor would their existence affect the merit of any of the work I've done as a writer, a philosopher, and so on. Nor would it restore the authority of any popes or priests, whose authority depends on the truth of the actual religions they profess.
If the deist or pantheist god turns out to exist after all, I can live with it. Insofar as some Christians these days are really just deists or pantheists, I don't see why I should try to "convert" them to my atheist position on the deist and pantheist gods. It doesn't really matter that much.
The gods that matter are the ones that want to interfere in our lives, the ones whose bidding is carried out by all the presbyters and pulpiteers. I'm talking, in particular, about the Old Testament Yahweh or Jehovah, the Trinitarian godhead of orthodox Christianity, the deities of orthodox theologians and popular preachers, and about Allah, as depicted in the Koran. We can confidently deny the existence of all those gods, and the authority of anyone who claims to be expert in what those gods think about what we should do and how we should live.