adjective -- prone to examining beliefs critically and skeptically (neologism based on the common misspelling of 'atheist' as 'athiest')
If you accept most beliefs unquestioningly, you're not very athy. If you do so with most beliefs, yet have a certain set of beliefs you refuse to examine critically, you're athier. If you apply critical examination to ALL beliefs, then you are athiest.
(Not to be confused with the noun 'atheist', which means 'somebody who does not believe in god(s)'.)