atheist vs atheistical

atheist is a noun but atheistical is not a noun.

Word	Noun	Adjective	Verb	Adverb
atheist	Yes	Yes	No	No
atheistical	No	Yes	No	No

As adjectives, atheist and atheistical are synonyms defined as:

atheist and atheistical: related to or characterized by or given to atheism

Other synonyms of atheist include atheistic.

atheist (noun)	atheistical (noun)
someone who does not believe in the existence of a god

atheist (adjective)	atheistical (adjective)
related to or characterized by or given to atheism	related to or characterized by or given to atheism
	rejecting any belief in gods