Perhaps that anyone of any religion is not also atheist.
Anyone in any theology believes in only their deity, but doesn't believe in the hundreds of other recorded, historical deities.
Even the theists are atheists.
A-the-ism. "A", lack of/without. "The(o)", god. At it's core atheism means that you don't believe in god, any god. So no, believers can't be atheists because they believe in at least one god.