Relating to antitheism; actively opposed to the belief in God and religious doctrine.
From antitheistic + -al (adjective suffix). This is a more formal, archaic variant of antitheistic that follows the pattern of adding -al to create emphasis.
English historically added -al to adjectives for emphasis or formality (like 'historic' becoming 'historical'), so 'antitheistical' was the old-fashioned way to say antitheistic with extra gravitas.
Complete word intelligence in one call. Free tier — 50 lookups/day.