The mug is inspired by Flew’s telling of the Parable of the Invisible Gardener. In it, one person holds there is a gardener. They camp at the garden, set up the a fence, electrify the fence and get bloodhounds, but there is no sign of the gardener. In the end, another person asks what the difference is between an invisible, senseless, noncorporeal gardener and one that doesn’t exist. Flew uses the parable for falsification and that religious language is meaningless.
You can find out more about the Parable of the Invisible Gardener here.