Shamiri said
"Is God willing to prevent evil, but not able? Then he is not omnipotent. Is he able, but not willing? Then he is malevolent. Is he both able and willing? Then whence cometh evil? Is he neither able nor willing? Then why call him God?"

This is pretty standard from atheists and here is the response I have heard from a RC Bishop and another a leading Muslim cleric. Funny thing is that the responses were delivered in different languages at different times, yet the content was nearly same :)

So here goes;

God put humans on earth to exercise their free will. He provided food, shelter and best of all guidance in form of prophets and scripture (Bible, Quran etc.).

Humans are free on this earth to make their choices; good or evil. Evil begets evil and thus creates a spiral of escalation that can only be broken by good and pious deeds.

end.

So what do the atheists say to that?