1. I'd like to say a bit more about the "evil-god" challenge. It's been popularized by Stephen Law, but he didn't originate the argument. Other atheists like Peter Millican, Christopher New, Edward Stein, and Charles Daniels have toyed with that argument.
The basic idea is for an atheist to concoct a thought-experiment in which he postulates an evil god that has the same explanatory power as the Christian God (or the equivalent). Millican dubs the two candidates God and Antigod respectively.
If successful, the idea is to neutralize theistic proofs, for even if theistic proofs are otherwise strong arguments for God's existence, because Antigod mimics God, the theistic proofs are equally consistent with the existence of an evil God. An atheist doesn't even have to directly evaluate or critique theistic proofs. He can concede, for discussion purposes, that these are good arguments. But unless they can discriminate between God and Antigod, they don't count as arguments for God.
2. There are two ways of responding to the evil-god challenge. One way is to demonstrate a flaw in the argument. To show that the evil-god hypothetical doesn't have the same explanatory value as Christian theism. The two positions are not systematically symmetrical.
3. However, I don't think the onus is on Christians to disprove the hypothetical. We can just shrug it off.
i) For one thing, there's a difference between paper doubts and real doubts. Just because you can imagine a delusive scenario isn't a rational basis to be skeptical. Humans have the ability to devise mind-traps. Concoct imaginative scenarios in which an illusion is indistinguishable from reality. But other than illustrating the limits of what's provable or disprovable, I don't see the point of thought-experiments which propose scenarios in which we cant know what reality is like. Suppose the thought-experiment is successful? What does that accomplish?
ii) In addition, global skeptical hypotheticals are paradoxical. An atheist is implicated in the same hypothetical. If Antigod exists, then the atheist is just as deluded as the Christian. Indeed, the evil-god argument is, in itself, part of the global illusion, foisted upon us by Antigod. It keeps us off-balance. Keeps us guessing.
iii) If reality is unknowable, what are we supposed to do about it? What purpose does the hypothetical serve? It has no affect on anything one way or the other. What you believe or disbelieve makes no difference. It's a kind of epistemic fatalism.
I'm mean, the thrust of these hypotheticals is not, "How can you know that you're not a brain-in-a-vat, trapped in the Matrix, or deluded by the Cartesian demon?" but, "You can't know that you're not a brain-in-a-vat, trapped in the Matrix, or deluded by the Cartesian demon!"
Suppose we dream up a radically skeptical thought-experiment that we can't disprove. Where do we go from there? Nowhere!
It's like being told that you're caught in a time warp. But if you are caught in a time warp, there's nothing you can do to break the vicious cycle. You don't remember the last time warp, so you can't do anything different this time around to break out. Indeed, each time the cycle repeats itself, you're told that you're caught in a time warp. That, in itself, is factored into the time warp.
iv) What does Law think his challenge is supposed to achieve? He's generated a self-dilemma. If his argument is successful, then there's nothing we can do in response to his argument since we can't outwit Antigod.
On the face of it, the purpose of his argument is to make people doubt Christian theism. He deploys the argument to influence belief. To change what people believe about Christian theism. To dissuade them from believing Christian theism.
But his argument is self-defeating. If his argument is flawed, it proves nothing. If his argument is sound, it changes nothing. For it puts us at the mercy of Antigod. There's nothing we can do to overcome the illusion. We can't even recognize the illusion.
An atheist believes there is no deity, but if the argument propounded by the atheist is sound, the atheist is hopelessly deluded! Both he and the Christian are in the same boat to nowhere.