That's agnosticism, not atheism.
This is the issue of answers, not the existence of the deity of your choice (although in that sense, a Christian is only one step away from being an atheist). Both agnostics and atheists have to admit they don't know everything, that there are things that will probably never be known in their lifetimes, if ever.
The religious can rest in the belief that even if they don't know something, their deity knows everything. If they don't know it they can always make it up and assign it to the knowledge of the deity, thus making it truth.