Theism is opposed to
atheism (of course), but neither of those two opinions exclude
agnosticism - which is more an
intellectual attitude than a true belief.
The most common case in the modern world is
agnostic atheism : people who do not believe in God, but who know they have no way to be totally sure that
God does not exist.
Of course, this may be slightly less true in countries such as
Iran,
Afghanistan and the
USA.