He who makes a claim bears a burden to demonstrate the truth of his claim. Theists have a burden to demonstrate their claim that God exists, and atheists have a burden to demonstrate their claim that God does not exist. Nowadays, however, it’s common for atheists to claim that the theist alone bears a burden of justification. They try to escape their own burden of justification by redefining atheism from a “belief that God does not exist” to “the absence of belief in God.” Since only positive beliefs can be defended, they are off the hook. All the pressure lies with the theist.
While I think their attempt to redefine atheism is intellectually dishonest, let’s grant the validity of their redefinition for a moment. Greg Koukl observed that while it’s certainly true atheists lack a belief in God, they don’t lack beliefs about God. When it comes to the truth of any given proposition, one only has three logical options: affirm it, deny it, withhold judgment (due to ignorance or the inability to weigh competing evidences). As applied to the proposition “God exists,” those who affirm the truth of this proposition are called theists, those who deny it are called atheists, and those who withhold judgment are called agnostics. Only agnostics, who have not formed a belief, lack a burden to demonstrate the truth of their position.