Atheists don't have anything to apologize for.
The questioner asked for more details by stating the following:
//if there is no belief involved, it is called agnostic. Atheisn [sic] is belief. Opposite of belief is doubt. If you are caliming [sic] to be "certain" of non-existence of God, you have a "belief". If you are not sure, then you are "agnostic"//
My response to these claims went as follows:
Atheism is the position one takes when one does not have a belief in a god. Now, you must not forget that "not having a belief" is not a belief. It does not mean that we "strongly believe" in the non-existence of god.
For example, think about this. Here's what a unicorn looks like [roughly] in most imaginations:
|An artist's impression of the pink unicorn|
Now, if unicornists are people who actively believe that this mythical creature actually exists in reality. Aunicornists on the other hand would be people who do not actively believe that this creature exists.
Note that aunicornists don't have to spend their time and energy maintaining a "belief" in the non-existence of unicorns, because if that was necessary, we will all be drained of our energy in maintaining beliefs about the non-existence of an infinite number of things! In contrast, all the aunicornists do is to not hold a belief about the existence of the unicorn.
Now the other thing is "agnostic". Contrary to popular belief, an agnostic is not a fence-sitter between an atheist and a theist. A person can be an agnostic atheist, or an agnostic theist.
Gnostic-Agnostic merely means whether one has knowledge or does not have knowledge about something. In the context of theism, that 'something' defaults to the existence of god. But one can also be said as agnostic about unicorns. It merely means he does not have any knowledge about the existence of unicorns.
An agnostic unicornist would be someone who does not have any knowledge about existence of unicorns, but continues to believe in their existence. An agnostic aunicornist would be someone who does not have any knowledge about existence of unicorns, and also does not believe in their existence.
Now, we cannot realistically have a gnostic aunicornist, because if we have verifiable knowledge about the existence of unicorns, it would actually be absurd to not believe in their existence.
The other thing is the apparent confusion between possibility and probability. Many times, theists claim that [at the very least,] it's possible that a god may exist, and many atheists would actually agree to that. I myself would say that it's possible that there actually is a god.
But possibility does not mean it's highly probable. In fact, possibility has no bearing on probability. Even with how much our total sum of human knowledge has expanded, the probability of god's existence is actually remaining very very very low. This is because of the total lack of evidence in support of god's existence, save some holy books and some anecdotal evidence. (Science does not accept anecdotal evidence as valid for good justifiable reasons.)
It is due to this extremely low probability that atheists reject the idea of god. But no atheist is ever certain that there is no god. We accept that there is a possibility that besides the deafening lack of evidence, there could be a god. But, the odds of that being true is so very low that we are able to live our practical lives as though there is no god.
By the way, please do not think that just because I'm giving a long-winded explanation here, it means I'm apologetic. I'm not any more apologetic about atheism than a physics teacher would be [(about the theory of relativity)] when explaining the theory of relativity to his students. Imagine him saying "I'm sorry about the theory I'm teaching you..." LOL!