There are three possible positions with regard to the proposition that there exists a god or multiple gods theism, atheism, and agnosticism. Arguably all three of these positions can take on the guise of religion if they take the form of adhering to dogma about it rather than concentrating on the logic underlying the issue. The key thing that makes something a religion is that it is based on faith rather than evidence and atheists can just as easily fall into the trap of believing that god or gods do not exist, dogmatically, without bothering to look at the evidence in any given case.
In terms of the logic that they employ what is the difference between the three groups?
Atheists, of course, are those who argue that the proposition asserting the existence of god or gods is false, whilst theists argue that the proposition is true, and agnostics are those who argue that the proposition has not been proven true or false yet. Each position may be seen as the product of perfectly rational reasoning processes based on different assumptions about logic itself.
Theists suggest that not only is the god or gods hypothesis a reasonable one, because of what they see as the evidence of creation all around us but that this amounts to such overwhelming and conclusive evidence that it is therefore correct to conclude that god or gods exist. Atheists would counter this by suggesting that the proposition in question should not be afforded the status of a reasonable hypothesis, to begin with.
Agnosticism, whilst accepting that any given theism is probably false, suggests that the proposition in question, i.e. that god or gods exist, cannot be proven true or false and that therefore, logically, we should neither conclude it to be true or false.
But rather we should wait for conclusive evidence one way or the other. Atheists reply that if something is not a reasonable hypothesis to begin with, because there is literally no unequivocal evidence to support it, then you should simply conclude that it is false, which counters the agnostics.
Some might see this last point as leaving the atheists in a position where they are attempting to argue against theism from a dogmatic religious’ position of their own that denies the god or gods hypothesis on the basis of no evidence rather than confirms it on the basis of no evidence. Actually, this is false. The atheist position is an argument about how we should deal with unreasonable hypotheses. It is not about attempting to affirm the truth or falsity of something that you do not have the evidence to prove one way or the other.