The problem of evil argument is one of the toughest arguments against theism. It's also one of the oldest going back at least as early as classical Greece and the time of Epicurus. The basic form is like so:
Is God willing to prevent evil, but not able? Then he is impotent. Is he able, but not willing? Then he is malevolent. Is he both able and willing? Whence then is evil?
Basically saying that if God is omniscient, omnipotent, and benevolent then evil should not exist in the world. Yet evil does, therefore that conception of God (typical theistic belief) does not exist. Hume weakens the argument somewhat by saying that although that conception of God may exist, the state of the world gives us no reason to come to that conclusion.
Anyway, there are a number of responses to this argument. Many theodicies, some not. But the one I'm going to focus on here is the most common proposed by theists: the free will response.
The basic free will argument is that God gives man free will and that it is man who chooses badly, that man is responsible for evil, not God. Some might object: But why is free will so great? Is the high price of evil worth free will? And the theist responds: Indeed, God creating man without free will is pointless. The very purpose of creation is for man to choose God and the way of God. Plantinga even posits that man without free will is a logical contradiction.
Now, I had heard this and I thought I came up with a good counter-argument to the theist. However, although I did come up with it independently, I was not the first to think of it. My argument attacks the assumption that the existence of free will in man automatically means that some men must choose evil. It is not contradictory to think that all men can have free will and that none will choose evil.
But how? By brainwashing them? No. Because as John Hick has shown, even if a person thinks he is free in that way, he is in fact enslaved to a will other than their own. A person might think he wants to run around and act like a duck, but really it is the hypnotist who controls that desire.
My argument is that as God is omniscient, He knows before he creates any person how that person will act. He knows if a person will be good or bad before he even brings him into existence. Why can't God, with his foreknowledge of how people will act, only create those who do good? Free will is still maintained - but only those who would choose well are created. They are not compelled to choose that way, it is their free choice to be good. So now that free will has been maintained and the necessary evil from free will debunked, the free will objection to the problem of evil is no longer useful to theists.