A/gnosticism pertains to what you know or claim to know; a/theism to what you believe or claim to believe. If they claim to know that leprechauns don't exist, or that their pet god does, ask them how they know that.
At the age of five, Skagra decided emphatically that God did not exist. This revelation tends to make most people in the universe who have it react in one of two ways - with relief or with despair. Only Skagra responded to it by thinking, 'Wait a second. That means there's a situation vacant.'
