Different people, for different reasons, challenge Christians with the "problem of evil" - if God is as good and powerful as the Bible tells us, then why is there so much evil in the world?
Ultimately, only Christianity can provide the grounds for a "problem of evil". Under the atheist, materialist world-view, there is no evil. Evil is an abstraction of thought. It is a value judgment. In a materialist world, those have no objective value. They are merely the swirling around of chemicals over here and over there. The "opinions" that they "experience" do not mean that evil is a real substance, with a real existence. The problem of evil? There is no problem, because there is no evil.
And yet it is indelibly impressed upon our souls that there is evil. And in as much as we know that, we also know that atheism and materialism are utterly defective and wrong.
However, if non-believers understood Christianity better, then they'd see the "problem" which Christians begin to see more and more as they grow. Namely, the problem of goodness. If we are as sinful and depraved as the Bible says we are - and we grow more convinced of that as we grow as Christians - then why does God keep on being so good to us? How can a God who is himself utterly and infinitely separated from evil, pour out such daily goodness upon us? Why does he not send the stinging lash of angry chastisement, and keeps on sending us mercy, kindness and grace? Why am I not damned? Why am I a co-heir with his wonderful Son?
The unified solution to the problems of both evil and goodness is found in the cross of Christ.