In a manner that opposes theism or belief in God; from an antitheistic perspective or position.
From antitheistical + -ly (adverb suffix). This adverb form allows antitheistic ideas to be described as being done or presented in a specific way.
This adverb form is so specialized it's rarely used today, but it shows how language can create very precise tools to describe how ideas are presented—'spoken antitheistically' means spoken from an antitheistic viewpoint.
Complete word intelligence in one call. Free tier — 50 lookups/day.