Philosophy of religion a bit more complicated than a dictionary,There are only three positions POSSIBLE regarding existence of god
1. God exists = theist.
2. God doesn't exist = atheist.(absence of god, not absence of belief of god)
3. Don't know = agnostic.
That is how majority use the words theist, atheist and agnostic.
Dictionaries should be updated, to keep things simple and clear and unambiguous.