English
aatheist
From a- + atheist, a negation of a negation.
The story
Technically (and disregarding this word's informality and humorousness), it should have been “anatheist”, with the euphonious prevocalic alpha privative (i.e., an-), from Ancient Greek ἀν- (an-); compare anaesthesia (from an- + aesthesia), anarchy, anorexia, and anaerobic.
Today it means
noun A person who does not believe in the existence of atheists.
Language periods are conventional ranges, not dates for this word.
Etymograph / aatheist