Some Christians accuse Richard Dawkins for declaring himself as an atheist. They , while claiming that they know everything but just with an additional line 'God told me so', accuse him for being 100% certain about a thing that he cannot know. Even some self-called 'agnostics' blame those who call themselves atheists.
There is some misunderstanding.
Define what you mean by 'God' first. If you mean Yahweh or Jesus, Richard Dawkins and I are 100% sure that they are made up characters. But if you mean some general great being that humans have never contacted before, Richard Dawkins himself said he think there is some low possibility. Why not? He does not yet have any data from outside our universe. I also think that kind of 'God' is possible. I think majority of atheists think like me.
In short, in most cases, atheists are atheists for all sorts of Gods that humans have been worshipping, and agnostics for beings outside our universe. So, we can be both atheists and agnostics. They are not contradicting.